A large part of music theory is talking about chords and chord progressions. So first of all, what actually is a chord?
By chord, we are normally talking about a basic triad, which as the name suggests is simply a collection of 3 notes. Usually it is the first, third, and fifth note from any given scale.
In the key of C major, these notes are C, E, and G. So this chord is a C major chord.
Or in the key of D major we would get:
Note that in D major we have an F#.
What order these notes appear in doesn’t matter, so the chord below is still a C major chord because it has the notes C, E and G.
Chords are said to be in different inversions depending on which note is in the bass (at the bottom). If the first note of the scale (the root note) is at the bottom, it is said to be in root position. If the 3rd is in the bass it is a first inversion, and if the 5th is in the bass it is a second inversion.
Describing different chords within one key
When talking about chord progressions (a series of 2 or more chords) we can just refer to them by their names, C major, G major etc. However, that doesn’t really give any idea of how they relate to each other. Instead, we number the available chords within a key, so we can see how they interact. For example, in the key of C the first chord, chord one, is a C major chord. If we then create a chord starting on the second degree of the scale D we get a D minor chord – this is because we are still in the key of C major, so the F is a natural, so in the key of C we have the following chords (usually roman numerals are used to number them):
This is the advantage of using numbers to describe chords, because we see the relationship between them without having to worry about what they are called. So a chord progression of I – IV – V – I will be essentially the same in any major key.
Hang on, what did dim mean on that diagram?
Good question; chord VII is always a little problematic because it is not a normal chord. In the key of C, chord VII has B, D and F which is neither a major nor minor chord. B Major should have both D# and F# and B minor would have an F#. We describe the chord as diminished (dim for short) because it is a minor chord where the 5th (the F# in this case) has been dropped by a semitone (diminishing the gap between the root and 5th) because of this chord VII is not commonly used.
Adding notes to a chord
So far we have just been using basic triad chords, however sometimes extra notes are added to the chord. The most commonly added note is the seventh. If we take a C major chord and add the 7th note from the scale we get:
This forms a C major 7th chord where the 7th (B) is a semitone below the octave (C). It is unusual to add a seventh to chord I, except in Jazz. In classical music it is commonly chord V which has the seventh added – note that when we add the seventh (F) to chord V, it is a tone below the octave (G) not the semitone we had above.
One final common adjustment to a basic triad is the suspended 4th, Here the 3rd degree of the scale is raised, so in the case of a C major chord the E is raised to an F giving us this chord:
As we can hear in this audio example