Some chords are very close relatives to others in terms of the notes they contain. Some chords can even share exactly the same notes and therefore have two or more different names. These chord relatives can be useful to know for substitutions and also for melody purposes.
Consider the chords C major 7 and E minor 6. Lets look at the notes they contain:
Notice both chords contain the same notes, and can therefore be used inter-changeably in both harmonic and melodic situations.
So, a major 7 chord contains the same notes as a minor 6 chord whose root note is one third higher.
Also consider the chords G major 6 and E minor 7. These two chords share the same notes also:
So, a major 6 chord contains the same notes as a minor 7 chord whose root note is one sixth higher.
Here are some other chord synonyms:
G major 7 contains the notes G (1st), B (3rd), D (5th) and F# (7th)
E minor 9 contains the notes E (1st), G (b3rd), B (5th), D (b7th) and F# (9th)
If we omit the root note of the Emi9 chord, these two chords share the same notes.
G7#5 contains the notes: G (root), B (3rd), D# (#5th) and F (b7th)
Db9b5 contains the notes: Db (root), F (3rd), G (b5th), B (b7th) and Eb (b9th)
If we omit the root note of the Db9b5 chord, these two chords share the same notes (D# is the same as Eb).
These chord relatives are by no means the only ones. Often you will find you can construct many other chord triads from the notes of a single chord, especially when we are dealing with extensions. It is therefore vital the guitarist knows the notes that make up any one chord. As an exercise, see how many chords you can make from an E13 chord.