Slash chords are used to indicate:
an inverted chord – where a note other than the root is sounding as the lowest note of the chord
G/B, C/G, Em/B etc
a chord which contains a note which isn’t part of the harmony used as the lowest note of the chord
G/F sharp, C/G sharp etc
Looking at each of these in turn.
When a chord is in root position (where the root of the chord is the lowest sounding part) we typically say that we are playing a C chord or F minor chord or the like. When one of the other notes of the chord is the lowest sounding part we should indicate this by stating that the chord is in first inversion – if the third of the chord is the lowest part, or second inversion, if the fifth of the chord is the lowest sounding part.
There are different ways of indicating this on paper. Probably the most common way for guitarists is to write the chord which is being played and then indicate the note which is the lowest sounding part. For example, a C major triad
in first inversion
would have the third of the chord in the lowest part. Using slash chords this would be written – C/E; the letter to the left of the forward slash indicates the chord while the letter to the right indicates the note which is ‘on the bottom’ of the chord. This can also be used for seventh chords:
A C major seventh chord
in third inversion (seventh chords can be in third inversion because of the extra note)
Would be written C/B.
Slash chords can be handy when explaining inverted extended chords. For example, a C13 chord, C E G Bflat D F A, with the thirteenth (A) in the lowest part is not usually referred to as being in ‘sixth inversion’. Written as a slash chord C13/A clearly (if we know the notes contained in the chord) indicates which note is the lowest sounding part.
For indicating lowest sounding notes which don’t belong to a chord, the principle is exactly the same:
the name of the chord is to the left while the note ‘on the bottom’ of the chord is to the right. If we have a G major triad GBD but we’re playing a C sharp in the lowest part it would be written G/C sharp
Or, if we had an A minor triad with a B on the bottom, it would be Am/B
Chord synonyms are chords which can be spelt in different ways. For example, a Bm7 flat 5 chord (which can also be called a B half diminished seventh chord) contains the same notes as a Dm6 chord
Or, if we look at a C11 chord
C E G B flat D F
there are other chords contained in this extended chord: C9, B flat major, E minor seven flat five, G minor etc. Because of this fact, and also the fact guitarists are unable to play all of these notes together, there will be times when certain groups of these notes may not sound like an eleventh chord – even though they all belong to the C11. As with many things in music it is the context which dictates the correct label to attach to a chord.
The C11 belongs to the dominant group of chords, which also includes C9, C7 etc. As their name suggests, dominant chords often move to chord I, as in this Perfect Cadence
In the above context the C11 can clearly be thought of as behaving in this dominant function.
How about in the following context?
Now the C11 shape sounds like it is part of the pre-dominant harmony preceding bar 2. In fact, because of the walking bass line, the C11 shape simply sounds as part of the B flat major seventh chord on the final beat of bar one.
Slash chords too, can often have dual, or multiple labels. If we consider again our C11 chord from beat three of bar one in the above example, it can be thought of as a B flat major chord with a C in the bass: B flat/C.