Interested in learning how chords are constructed, rather than just relying on remembering shapes on the fretboard? As the third article in our series on guitar theory for guitarists our introductory guide to guitar chord theory explains (in simple terms) how to read chord charts, how chords are named, and how they are constructed, plus a whole lot more. So if you have ever wanted to understand the music theory behind chords, stay tuned!
What is Guitar Chord Theory?
Guitar chord theory encompasses how chords are constructed from scales, and the different qualities (major, minor, etc.) and types (triads, 7th chords, extended chords) based on the intervals of the chord tones and the number of notes used to build them.
What are chords?
Any combination of three or more different notes played at the same time, usually in support of a melody.
This doesn’t mean playing any combination of three or more notes played on guitar will sound musical. But, provided you are playing a combination of three or more different notes (e.g. not the same notes in different positions on the fretboard) at the same time (e.g. strumming), you are technically playing a chord.
What are Arpeggios?
If you understand the concept outlined above, you also understand arpeggios. An arpeggio is simply a chord with the notes played sequentially e.g. one at a time, rather than strummed.
Should you Learn Chord Theory?
There are two opposing schools of thought for guitarists when it comes to learning any aspect of music theory, with compelling arguments supporting each, including:
- Music theory isn’t necessary and can stifle creativity
- Music theory helps you better understand the music you are playing and allow for a more expansive approach to guitar
At the heart of both arguments lies the question, what is more important, technical knowledge or feel?
But, depending on your point of view, it’s probably the wrong question to be asking.
Most guitarists understand music theory isn’t strictly necessary, but who’s to say that learning the technical aspects of the guitar, and music, in general, equates to the guitarist having less feel for their instrument or being less creative? Just because you understand the rules, doesn’t necessarily mean you are strictly bound to them.
In most cases, the choice will come back to the individual and their personal goals on the guitar. Personally, I find it useful when writing, working on a chord progression, or just jamming with friends to understand how the chord has been constructed rather than learning the specific chord shape.
It’s helped me become a more rounded musician and songwriter. Having said that, I spent many years not knowing any music theory. Instead, I just referenced fret numbers instead of notes, and while this worked, looking back it was harder than it needed to be.
So while not strictly necessary, it’s my belief that if your aim is to become a musician, learning music theory and understanding how chords and scales relate to one another on the guitar can only be an advantage.
Open and Barre Chords
Most guitarists will start out by learning open position chords. Open position chords are chords that include open strings (unfretted strings) and are usually played in the top three or four frets as per the example of an open C Major on the left.
Open chords (aka cowboy chords) are an important first step. The different shapes help develop muscle memory and finger strength. And, many of the shapes are then modified to form barre chords, particularly the open A and E shapes used for open chords.
Barre chords (also known as closed position chords) do not contain open strings and generally require the strings to be ‘barred’ across one fret. The index finger is used to replicate the nut, preventing open strings from ringing out.
Barre chords are generally more difficult for beginners, especially on the acoustic guitar, and require decent finger strength and dexterity.
We’ll cover open and closed position chords in more detail when discussing voicings a little later on, but keep in mind there are many different ways to play chords on the guitar.
This becomes apparent when you consider there are only 12 notes in the chromatic scale (the chromatic scale includes all 12 notes in western music) and most guitars have at least 120 frets.
How to Read Chord Charts
Chord charts demonstrate how a guitar chord is intended to be played. The following chart shows ‘A major’ in the open position.
The chart itself is essentially an image of the fretboard. The bass strings are on the left and treble strings on the right. If you are a right-handed guitarist, this represents the fretboard if you were looking at it directly. Some charts may be presented in a horizontal layout, meaning the chart is turned 90 degrees anti-clockwise however these are generally less common.
The vertical lines represent the strings. The horizontal lines represent the frets. The dots represent the finger placement on the fretboard. The root note is sometimes shown using a different color combination, but as in the chart above, not in all cases.
The ‘X’ above the first fret indicates the string is not to be played, while the ‘O’ indicates the string is to be played but is unfretted, or in other words, it is an open string.
Lastly, the numbers at the bottom of the chart represent the fingers used to fret the notes. In this case, the 2nd (middle finger), third (ring finger), and fourth (pinky) are used to play the A major chord. Some chord charts include numbers representing finger positioning, in most cases, you won’t find finger positioning included on acousticguitarist.com but I have included it as an example in the chart above.
Chord charts are not just used to represent open chords. Below is an example of an A barre chord chart.
Being a closed position chord, the 5th fret is barred which is indicated by the line connecting the dots representing the finger positions (this is often a curved line above the top fret). Unlike an open position chord, the chart itself is a representation of a section of the fretboard taken from the 5th fret, indicated by the ‘5’ marked on the left side of the chart.
You will also notice the number 1 used on the notes played at the fifth fret. This means all the notes that align with the fret are fretted using the first (index finger).
How are chords named?
Chords are named based on the root note and the quality of the chord it is. For instance, F Major takes its name due to the root note being ‘F’ and being of ‘major’ quality. Below is an example of a D Major.
The root note defines the chord. For example, D is the root note of all D chords (e.g. Major, minor, augmented, suspended, etc.).
C is the root note of all C chords and so on. The root note is the first note of the corresponding scale, however, when referring to scales e.g. the minor scale, pentatonic scale, etc. we call the starting note the tonic.
*It helps to have a basic understanding of guitar scale theory, which you can read more about here.
In the majority of cases (but not always) the root note is the first and lowest-pitched note. Chords that consist of the root note being the lowest pitched note are called ‘root position chords’.
For instance, when playing an open position D Major, the lowest note played is the open D (4th string) as the A and low E strings are not played (as indicated by the ‘X’ above the 5th and 6th strings.
In the majority of open position chords, the root note will be a note played on either the low E, A, or D strings.
Another good example of this is open A Major. The first string is the open A string which is also the root note.
Many chords include more than one root note. For example, E major consists of both the open low E and high E string played as individual notes. This isn’t just the case for root notes, notes are often repeated in chords. For instance, the following are both C major as C major is built from C (root), E, and G.
While the first shape contains just the three notes, the second example utilizes 6 notes, however only C, E, and G are required, so in the second example, the notes are repeated. The second example would be referred to as a slash chord, as the lowest note is G (3rd fret of the 6th string) and would usually be written as C/G.
Chords shapes that use repeat notes often do so to make the chord easier to play (consider the muted strings in the first example) and also tend to sound richer, this becomes even more apparent when playing in drop tunings.
Regardless of your experience with music, you will most likely have heard the musical terms ‘major’ and ‘minor’. ‘Major’ or ‘minor’ is a way of describing a chord’s ‘quality’ and relates to its distinctive sound or flavor.
For example, major chords are often described as sounding happy. Minor chords are often described as sounding sad or more serious than major chords. Suspended chords, on the other hand, don’t resolve to major or minor, and therefore sound tense, and unresolved.
Chords are not restricted to only major, minor, and suspended, however. The table below shows a list of the four most common qualities and a description of their sound.
|Major||Maj||Happy, simple, resolved|
Other less common qualities include, but are not restricted to:
|Suspended fourth||sus4, (sus)|
|Suspended second||sus2, (sus)|
|Minor added ninth||m(add9)|
|Minor sixth||M6m (-6)|
|Sixth, added ninth||6/9|
|Minor sixth, added ninth||m6/9|
|Minor seventh||m7, min7, -7|
|Diminished seventh||dim7, dim|
|Seventh, suspended fourth||7sus4, 7sus|
|Minor, major seventh||m(maj7), m(+7)|
|Major seventh, flat fifth||maj7♭5, maj7(-5)|
|Minor seventh, flat fifth||m7♭5, m7(-5)|
|Seventh, sharp fifth||+7, 7(♯5)|
|Seventh, flat fifth||7♭5, 7(-5)|
|Seventh, flat ninth||7♭9, 7(-9)|
|Seventh sharp ninth||7♯9, 7(♯9)|
|Major ninth||maj9, M9|
|Minor ninth||m9, min9|
|Minor eleventh||m11, min11|
Types of Chords
Triads, sevenths, and extended chords
Chords can be categorized into groups based on the number of notes they are constructed from and include triads, seventh and extended chords.
Triads are the most common chords and are constructed from three notes, each a third above the previous one. While you might think this strange that there are only three notes considering we play between 4 and 6 strings when strumming a chord, the three notes are often repeated (more on this shortly).
There are four qualities of triad:
- Major triad
- Minor triad
- Augmented triad
- Diminished triad
These four are arguably the most common chords played on guitar.
To construct major or minor triads the root, 3rd, and 5th notes of the corresponding scale are required.
Augmented triads are similar to major triads, utilizing the root and major third (3rd scale degree of the major scale) to construct an augmented chord, however, the 5th note of the scale is, as the name implies augmented e.g. raised half a step (semitone).
A Diminished chord (also a triad) is similar to a minor chord e.g. it utilizes the root, minor third (third scale degree of the minor scale), and a diminished 5th e.g. lowered half a step (semitone).
If this is confusing, don’t worry. We’ll discuss more about how chords are constructed further along but for now, a good way to get your head around chord theory is to consider chords as triads (containing three notes), with seventh and extended chords having additional notes added from a scale.
A seventh chord (7th chord) is simply a triad that includes a seventh interval. The dominant 7th is the most common on guitar and is simply a Major triad with a minor 7th interval, and tends to sound bluesy. This makes sense if you consider that blues tend to utilize the notes of the minor scale over major chords and it is often the tension between major and minor that we associate with a ‘bluesy’ sound.
Again, if this doesn’t make a lot of sense right now, don’t despair, this will become much clearer as we go along.
What are Scale Degrees?
Notes included in scales, for example, the C Major Scale (C – D – E – F – G – A – B) are assigned numbers, known as scale degrees. For example, E is the third scale degree, as indicated above, B is the seventh. Scale degrees are useful as we can use them to construct chords from scales in any key without knowing the individual notes needed.
Like 7th chords, extended chords are constructed from more than three notes, with any additional notes beyond the basic triad extending beyond the 7th scale degree. This means they include notes from an octave higher. Examples include 9th, 11th, and 13th chords.
An octave, if unaware simply means the interval between two notes of the same pitch, with one being half or double the frequency of the other. For instance, your open E string is technically the same pitch as the 12th fret of your E string but twice the frequency. You can read more here about why we hear notes an octave apart and recognize them as being musically equivalent here.
What about two-note chords?
You may have heard guitarists refer to two-note ‘power chords’. These are shapes that include the root note and the fifth scale degree only. They are neither major nor minor in quality as they do not contain a third scale degree which would otherwise determine a chord to be major or minor. For now, it is helpful to understand that any notes played together are considered harmony (the combination of one or more notes played at the same time) but for the notes to be a chord by definition they must contain at least three different notes. However, you may see two-note shapes referred to as ‘partial chords‘.
How chords are built
The chord scale relationship
Chords are built from scales. They literally go hand in hand when it comes to creating rhythm and melody. For example playing notes from a corresponding scale e.g. the E minor scale over an E minor chord will sound musical, or in key.
Below are the notes for some of the C, E, and G major scales and the notes that form the corresponding chord. See if you notice a pattern? We’ll revisit this shortly.
|C Major Scale|
C – D – E -F – G – A – B
|C Major Chord|
C – E- G
|E Major Scale|
E – F♯ – G♯ – A – B – C♯ – D♯
|E Major Chord|
E – G♯ – B
|G Major Scale|
G – A – B – C – D – E – F♯
|G Major Chord|
G – B – D
The main difference between scales and chords is how they are constructed with regard to the notes they comprise.
For example, scales utilize specific step patterns (not to be confused with scale pattern, which shows guitarists how to play scales with the least distance between notes on the fretboard) that determine their quality e.g. major, minor, etc. Step patterns are made up of whole (W) and half (H) steps. Whole steps represent 2 semitones (2 frets on the fretboard), while a half step represents a semitone or a single fret.
The major scale is derived from the chromatic scale (all notes included in western music) using the following step pattern:
Major scale step pattern
W – W – H – W – W – W – H
Chords can be constructed from scales using intervals (more on this shortly) or by using a formula based on the notes of the major scale.
Method 1: Scale Degree formulas and the major scale
We can build chords using formulas based on the major scale. For example, the formula for major chords is:
Major scale formula
1 – 3 – 5
In a practical sense, this means we take the first, 3rd, and 5th notes, or scale degrees from the major scale to build a major chord. Each individual note making up a chord is known as a chord tone. The tonic (starting note) of the scale determines the value of the chord. For example, the A major scale consists of the following notes:
The A Major scale
A – B – C♯ – D – E – F♯ – G♯
The first, or root note (scale degree) is the A, the third the C♯ and the fifth note is the E. This means the individual notes (referred to as scale tones or scale degrees) that make up A major are A, C♯ , and E.
Minor chords, on the other hand, may also reference the major scale but instead use the formula: 1, ♭3 and 5.
This means the third scale degree is flattened by a half step. This is useful in a practical sense as it means you can make any major into a minor chord by flattening the third a half step (semitone), which is simply one fret on the fretboard. All chords constructed using scale degrees reference the major scale in this way.
It’s also true that if you constructed a minor chord from the starting point of its corresponding natural minor scale, you would utilize the 1, 3, and 5 scale degrees of the minor scale, as seen in the example below:
The A minor scale
A – B – C – D – E – F – G
Based on this, the chord notes required to construct an A minor chord are A, C, and E.
Common chord formulas
Below is a list containing the most common scale degree formulas. Using the formulas below will allow you to construct your own unique chord voicings anywhere on the fretboard.
|Major||1 – 3- 5|
|Minor||1 – ♭3 – 5|
|Diminished||1 – ♭3 – ♭5|
|Augmented||1 – 3 – ♯5|
|Suspended fourth||1 – 4 – 5|
|Added ninth||1 – 3 – 5 – 9|
|Minor added ninth||1 – ♭3 – 5 – 9|
|Fifth (partial or power chord)||1 – 5|
|Sixth||1 – 3 – 5 – 6|
|Minor sixth||1 – ♭3 – 5 – 6|
|Sixth, added ninth||1 – 3 – 5 – 6 – 9|
|Minor sixth, added ninth||1 – ♭3 – 5 – 6 – 9|
|Seventh||1 – 3 – 5 – ♭7|
|Minor seventh||1 – ♭3 – 5- ♭7|
|Seventh, suspended fourth||1 – 4 – 5 – ♭7|
|Minor, major seventh||1 – ♭3 – 5 – 7|
|Major seventh, flat fifth||1 – 3 -♭5 – 7|
|Minor seventh, flat fifth||1 – ♭3 -♭5 – ♭7|
|Seventh, sharp fifth||1 – 3 – ♯5 – ♭7|
|Seventh, flat fifth||1 – 3 – ♭5 – ♭7|
|Seventh, flat ninth||1 – 3 – 5 – ♭7 – ♭9|
|Seventh sharp ninth||1 – 3 – 5 – ♭7 – ♯9|
|Ninth||1 – 3 – 5 – ♭7 – 9|
|Major ninth||1 – 3 – 5 – 7 – 9|
|Minor ninth||1 – ♭3 – 5 – ♭7 – 9|
|Eleventh||1 – (3) – 5- ♭7 – (9 )- 11 * (3) and (9) are optional|
|Minor eleventh||1 -♭3 – 5 -♭7 – (9) – 11 * (9) is optional|
|Thirteenth||1-3-5-b7-(9)-(11)-13 * (9) and (11) are optional|
What’s the deal with suspended chords?
Taking a look at the table above, you might notice suspended chords (along with power chords) are the only chords that do not include the third scale degree. Because of this, they are neither major nor minor, as the third determines whether a chord is minor (flattened third) or major. Suspended chords replace the third with a fourth (sus4), or a second (sus2) scale degree.
What are diminished chords?
A diminished chord is simply a minor chord with a flattened fifth. There are three types of diminished chords (as we can see in the chart above) Diminished triads, half-diminished, and diminished 7th chords.
Method 2: Using intervals
We can also construct chords from scales using intervals. Intervals can be ‘harmonic’ e.g. played at the same time, as is the case with chords, or ‘melodic’ meaning the notes are played sequentially. Before we can build chords using intervals, first we need to discuss the master of all scales, the chromatic scale.
The chromatic scale
There are 12 notes in western music, and these 12 notes make up the chromatic scale. This means every scale (along with every note) comes from the chromatic scale, and as a result, is helpful to think of the chromatic scale as the ‘master scale’.
Whole steps and half steps (whole tones and semitones)
The notes in the chromatic scale are separated by half steps. A half step is a distance between two adjacent notes e.g. A to A♯ is one-half step (1 fret on the guitar), equal to a minor second interval (more on this shortly). While a whole step (2 frets on the guitar) is twice this distance e.g. A – B.
This tends to make more sense when considering the layout of a piano. The first arrow indicates a half step, the second arrow shows a full step between the D# and F.
The white keys are whole notes e.g. C, D, E, F, G, A, and B. The black keys in between are sharps or flats e.g. C♯, D♯, F♯, G♯ and A♯
- When you see notes, denoted with a sharp symbol(#) or flat symbol (♭ ) but appear to be of equal pitch, they are known as enharmonic equivalents, meaning they are of the same note value e.g. a C♯ is the musical equivalent of a D♭.
Only 5 of the 7 notes have sharps or flats. This is why the piano does not feature black keys between each white key, which as it turns out is a good thing as it would be difficult to navigate the keyboard if the layout was exactly the same for every note.
Why are intervals important?
Half and whole steps are a type of interval e.g. the distance between notes. Half and whole steps are fine for discussing smaller intervals, but when the distance between notes is more than a whole note we can use additional terminology and this becomes more useful than simply discussing notes.
For example, if we consider an A major chord, the notes used are A, C♯, E.
However, if we want to really understand how major chords are constructed without referencing the major scale it’s more effective to learn the intervals than the notes themselves. The intervals for all major chords are the first (root), major third (four half notes from the root), and perfect fifth (7 half notes) taken from the chromatic scale. In the case of minor chords, we simply change the major third to a minor third (3 half notes).
This means you only need to know the intervals of each chord type rather than the individual notes, making it far easier to construct chords or adapt a major chord to a minor by simply flattening the major third to a minor third. This shouldn’t be confused with scale degrees, which are based on the major scale.
The difference between using scale degrees and intervals
While you may notice similarities, there are key differences between intervals and scale degrees. Scale degrees reference the major scale and are relative to the root note, meaning the root is counted as 1, because it is the first scale degree and the ascending scale degrees from the root (2,3,4,5,6,7) define the position of the note in the scale relative to the root note.
Intervals on the other hand define the distance between any two notes (if discussing the distance between two notes of a scale we use the term diatonic interval). The interval number refers to the number of whole notes or letters between the two notes.
As we can see in the table above the distance between F and A is three notes, so the interval between F and A is a third. To determine the intervals ‘quality‘ we need to establish how many actual notes, as opposed to whole notes the interval consists of.
In the table above we can see the distance between F and A spans three letters (F, G, and A), however, it spans four notes due to the G# between the G and the A. In a practical sense, this means the interval is a major third. If we were referencing the distance between the F and G# the interval would be a minor third.
How intervals relate to each other
There are 5 categories of intervals:
- Perfect: 5th, 4th, unison and octaves
- Major: 2nd, 3rd, 6th, 7th
- Minor: 2nd, 3rd, 6th, 7th
- Augmented: Half step added to a perfect or major interval
- Diminished: Half step reduced from a perfect or major interval
These categories relate to one another. For example, a major interval when reduced by one step becomes a minor interval.
|Distance Between Frets/Notes||Interval Name|
Alternatively, If we lower a perfect interval by one step, the interval becomes a diminished interval, and a major or perfect interval if raised a half step becomes an augmented interval.
Different intervals have different names. Below is a table listing the 12 intervals, the distance between the notes, and the interval name.
|Distance Between Frets/Notes||Interval Name|
|2||Major 2nd (Diminished third)|
|3||Minor 3rd (Augmented second)|
|4||Major 3rd (Diminished fourth)|
|5||Perfect 4th (Augmented third)|
|6||Tri-tone (Diminished fifth)|
|7||Perfect 5th (Diminished sixth)|
|8||Minor 6th (Augmented fifth)|
|9||Major 6th (Diminished seventh)|
|10||Minor 7th (Augmented sixth)|
How intervals are named
Intervals are named by their interval type and their interval number e.g.
Perfect intervals (P)
Intervals that are either unison, 4th, 5th or a whole octave are known as perfect intervals. They remain the same whether inverted. An inverted interval is an interval that has been turned upside down e.g. the starting note becomes the final note and vice versa.
Non perfect intervals
Non-perfect intervals are either major or minor intervals and include the second, third, sixth, and seventh.
Augmented intervals (A)
Augmented intervals are perfect or Major intervals that have been raised by half a step.
Diminished intervals (d)
Diminished intervals are perfect or minor intervals that have been lowered by half a step.
How major chords are built using intervals
As we know, major chords consist of the root note or first, the 3rd (the 3rd is major unless otherwise indicated), and the 5th (perfect fifth) intervals.
If we build an A major chord, starting on the A (the root note) we get the following notes:
A – A♯ – B – C – C♯ – D – D♯ – E – F – F♯ – G – G♯
- The first note is the A.
- A major third from the root note (4 half steps) is the C♯
- The perfect fifth from the root (7 half steps) is the E
This means A Major consists of the notes A, C♯ , and E.
Qualities and Intervals
Below is a list showing the 4 triads types we listed at the beginning of this article and their respective intervals.
|Major||Root – Major third (M3) – Perfect Fifth (P5)|
|Minor||Root – minor third (m3) – Perfect Fifth (P5)|
|Diminished||Root – minor third (m3) – Diminished fifth (dim5)|
|Augmented||Root – Major third (M3) – Augmented Fifth (Aug5)|
Now that we have a basic understanding of how chords are built, the different types and qualities of chords, and their relationship to scales, you might be wondering which chords go together?
I’ve written a complete article on this topic here, but to summarise, we use scale degrees to work out the root notes of each chord, and the quality of the chord is determined by the following pattern: Major Minor Minor Major Major Minor Diminished.
For example, if you are playing in a minor key e.g. E minor you may be wondering which chords you can use within this key.
E minor scale
|Chord||E Maj||F♯m||Gm||A Maj||B Maj||Cm||D dim|
The same applies when playing in a major key, as shown below.
E Major scale
|Chord||E Maj||F♯m||G♯m||A Maj||B Maj||C♯m||D♯dim|
What is a dominant chord?
This can be confusing as the term ‘dominant’ refers o two different things musically, although they do relate to one another. If discussing chord function (the relationship and movement between chords) dominant chords are chords built on the 5th scale degree.
If we look at a C Major scale for example:
C – D – E – F – G – A – B
The dominant chord in the key of C would therefore be G, as this is the 5th scale degree of the C Major scale, and therefore the root note of the dominant chord. The dominant chord introduces instability, which is resolved by returning to the tonic.
Dominant is also a term applied to specific 7th chords, that utilize a minor 7th interval added to a major triad. e.g. a C dominant 7th (aka c7) is a C major triad with a fourth note added, which is a minor 7th interval from the root.
What are extended chords?
Chord extensions are triads that include intervals past the 7th. This includes the 9th, 11th, and 13th chords. For example, a 13th chord includes the 9th, 11th, and 13th scale degrees. But on the guitar, this would mean we have the root, 3rd, 5th, 7th, 9th, 11th, and 13th which would equal 7 notes, and considering we only have 6 strings is impossible to play. In this instance, a note is usually omitted, in most cases the 5th, and in many cases the 9th and 11th.
What are chord functions?
e.g. what is an I-IV-V Chord Progression?
Chord function (aka harmonic function) describes the interaction or relationship between chords. It is influenced by genre and context to some extent e.g. the chord preceding and being played after. Chord function is usually broken down into three specific groups: tonic, subdominant, and dominant, and are represented by Roman numerals e.g. I, IV, V means we play the first, fourth, and fifth chords of a key.
Much of this depends on the notes within a given chord and mostly relates to movement within a chord progression e.g. some chords introduce instability that increases the need for the piece to resolve on the tonic e.g. a more stable chord within the context of the chord progression.
I’ve written a much longer piece here that describes chord function and how to know which chords work well together.
What are diatonic chords?
Diatonic means ‘in key’ so diatonic chords are simply chords within a given key.
What is a chord inversion?
The term inversion means to reverse position. In the context of chord theory, an inversion means we change the structure of the chord without changing the chord’s value e.g. the root is the lowest note of a chord usually, but if we voice the chord so that the third interval of the chord is the lowest note it would be known as a first inversion. If the fifth was the lowest note it would be a 2nd inversion chord.
What are slash chords?
If you see a chord written like this G/B The G indicates the chord e.g. G Major. The B indicates we are to add a B in the bass, making it the lowest note. In the case of a G Major chord, the B is the 3rd, however, if written as G/B we are instructed to make the B the lowest note in the chord, so the first inversion of G (as discussed in the answer above).
Not all slash chords are inversions, however, meaning we can also include non-diatonic (not of the same key) notes.
While there is far more that could be expanded upon with regard to chords, hopefully, this article has helped explain the basics of chord theory while avoiding getting too technical. I know from experience that once you have a basic understanding of how chords are built and how they relate to scales and the notes of the fretboard it can really help with writing and also learning songs.
If your focus is on becoming a better guitarist or songwriter the basic music theory covered in this series (learning the notes of the fretboard, understanding scales, and the information included in this article) covers the basics and is really all you need to know as a guitarist. However, the language of music is incredibly interesting and I’d recommend doing further research if you really want to take your understanding of music further.