Like triads, it is always possible to substitute a scale tone 7th chord which is two degrees further ahead in the scale. Shown below are the scale tone seventh chords in C major.
By substituting Em7 (chord 3) for Cmaj7 (Chord 1), a Cmaj9 chord is implied. By substituting Fmaj7 (chord 4) for Dm7 (chord 2), a Dm9 chord is implied. This process works for every tone of the scale. Also like triads, it is possible to keep substituting two ahead until you reach the 13th of the chord by substituting Bm7b5 (chord 7) for Cmaj7. For each degree of the scale there are three other possible scale tone 7th chords which can be used as substitutions. Over chord I these are 3, 5 and 7. The following example shows the scale tone 7th substitutions for chords 2, 5 and 1 in the key of C.