The term "extended chords" refers to all the chord types which contain more than four notes, most notably 9ths, 11ths and 13ths. They are all extensions of the basic chord types and can all be related back to the major scale. The next example shows the C major scale with degrees written under the notes up to the 13th. These thirteen notes are all used in chords in various combinations. They are all that is needed to create all chord types, each with its own unique formula.
C Major Scale up to the 13th Degree