Welcome to a short introduction of the theory behind chords. You will learn how a chord is built and what separates different groups of chords. See also general music theory.
In the cases there we only have a letter, such as C, it is a common major chord. A major chord consists of three notes: the 1st, the 3rd and the 5th notes in the scale. In what scale, you may ask. In the C major scale we have these notes:
C – D – E – F – G – A – B
For a C major chord we can locate the 1st, the 3rd and the 5th as C, E and G. If you're interested to learn guitar scales, see scale diagrams and more information.
Sometimes we find chords with names such as Cm, Dm and Em. The “m” stands for minor and in a minor chord we have three notes: the 1st, the minor 3rd and the 5th. The notes in Cm are C, Eb (E flat) and G.
In the majority of chords, the 3rd decides whether a chord is major or minor.
You may ask how it comes that we only have three notes in a C major chord when you have learned to strum five strings playing this chord:
C major chord
The reason is the instrument. The notes are ordered less symmetrical on the guitar's fretboard opposed to the piano keyboard. Another reason is that musical context differs between piano and guitar: you cannot play the harmony with your left hand and the melody with your right hand simultaneously on the guitar. Therefore, we don’t need to worry about too many notes being played at the same time.
As you can see in the diagram above, there're five notes played together in a C major chord, these are: C (5th string) , E (4th string), G (3rd string, played open), C (2nd string) and E (1st string, played open). It is the most practical way to play the open C chord.
In chords, we often find numbers after the letters: C5, C7, and C9 to list a few. In a C5 chord (also called power chord), we only have two notes: the 1st and the 5th. That's pretty unproblematic. Therefore, the C5 consist of C and G.
In the case of the seven, things are little messier. First of all, there are three groups: C7, Cm7 and Cmaj7. C7 is called the dominant 7th. A C7 consists of the C minor chord plus the flat 7th (but it's not a minor chord, neither a major). The flat 7th is a B flat (Bb). In a Cm7 chord we add the flat 7th to a C minor chord. Finally, in a Cmaj7 chord we add the major 7th to a C major chord.
You may wonder what actually differ between chords like C9 and Cadd9 or C13 and Cmaj13. Here are the answers from a tone perspective:
The difference between 9th and add9 is the flattened 7th.
C9: C, E, G, Bb, D
Cadd9: C, E, G, D
In other words, the 9th chord includes all the tones in a 7th chord and is extended with a ninth.
The difference between 9th and 7(#9) is a raised ninth
C9: C, E, G, Bb, D
C7(#9): C, E, G, Bb, D#
C7(#9) is an altered ninth chord, quite uncommon, but it is not impossible you will encounter it.
The difference between 13th and major 13th is a flattened and a natural 7th.
C13: C, E, G, Bb, D, A
Cmaj13: C, E, G, B, D, A
When you memorize this you are helped by thinking about the 7th versus maj7 and when add two notes to each chord (13 and 13maj).
On this site, you find lots of other chords like sus chords and slash chords. These chords have special sections and are explained together with its theory. You could get supplementary knowledge by reading What is a guitar chord?.
The connections of chords
Some chords are connected to each other in the way the notes belong to the same scale. For example:
C - Dm - Em - F- G - Am - Bdim
These seven chords have notes that all belongs to the C major scale. Try to play some of them in different order and you can hear that they fit well together.
The same thing could be done with four-notes chord types:
Cmaj7 - Dm7 - Em7 - Fmaj7- G7 - Am7 - Bm7b5
Once again, all these chords have notes that belong to the C major and once again you can play them and hear that they fit well together.
It is possible to include atonal chords, which is chords that doesn't fully match the notes in the key. One such replacement is VI7 instead of vi. For example, A7 instead of Am in the key of C major and a chord progression can look like: C - A7 - Dm - G.
See the Chart with chords sorted by key for an overview of the same thing in different keys.
Chords and intervals
The notes in chords are often referred to as intervals, such as a third or a fifth. It is important to understand the concept of intervals. Principally it is quite easy – you need to be able to count to thirteen and what you are counting are scales steps …
The relationship between intervals, chords and scales are illustrated by comparing a C Major scales and a C Major chords from the perspective of scale steps. You can see why the second and third note in a C chords (as well as other major triads) are referred to as third and fifth.
The first note is not called the first, which wouldn’t be illogical, but the root.
That is clear and simply, isn’t it? It can, however, be a little more complicated. You may encounter terms like both major and minor third, perfect fifth and major and minor seventh. Let’s take a closer look on by change the C Chord from the previous table with a Caug chord instead:
The augmented chord (abbreviated aug) consist by a root, a third and an augmented fifth. The augmented fifth are G# note, a G with a sharp (#) sign, and therefore outside the C scale. The correct scale steps for this note is written as #5, indicating that it is a tone one semi-step above G.
Here comes a third example:
The dominant seventh chord (abbreviated 7 or, less common, dom7) consist by a root, a third, a fifth and a minor seventh and as can be noted from the table doesn’t fully match the C scale. In C7, the minor seventh is Bb, a B with a flat (#) sign. The correct scale steps for this note is written as b7, indicating that it is a tone one semi-step below B.
As you have seen, it is not always enough to describe the intervals in a chord by third, fifth, seventh and so on. Sometimes, there are anomalies and in such cases the intervals are pinpointed by additional information like “minor” (third), “minor” (seventh) and “perfect” (fifth).
When omitting notes, strange things that can happen
A normal guitar has six strings, but a chord could include more than six tones. This is taking care of by omitting some of the least important notes and is not a big deal in most cases, even a pianist with ten fingers available often omit notes in four-notes chords and above.
But in some cases, odd things can happen. For instance, major 11th and minor 11th chords could be played identical on the guitar.
For example, this chord in short notations: X02030. As the chord is played, it includes the notes A, E, G, D and E. These tones are included in both A11 and Am11:
A11: A - C# - E - G - B - D
Am11: A - C - E - G - B - D
Because of the omitted notes, the "left-over" notes will be the same. Ideally, cases like these are avoided, but if you happened to see major and minor chords that are identical, you know why it could be.
For more in-depth reading, see: The Chord Theory ebook