Chord Substitution
If you analyze the notes of these scale tone 7th chords, you will notice that they are all based on the C major scale tone triads and each one has another 3rd interval added above it. Like dominant 7th chords, it is possible to leave out or more notes when playing all the other 7th chord types.
Because the left hand is often playing the root of each chord, it is common to leave the root note out of the right hand voicing. A common method of playing 7th chords it to use only triads with the right hand and play the root note with the left hand. If you choose the triad two notes ahead in the scale of the key you are working in, you can imply a 7th chord. This is called chord substitution. E.g. for a CMaj7 chord, play a C bass note with the left hand and an Em triad instead of a CMaj7 with the right hand. The combined notes are C, E, G, B, which is a CMaj7 chord. This could also be described as Em/C (a slash chord). Most slash chords add up to another chord type (e.g. some kind of 7th chord). Sometimes it makes more sense to describe the chord as a slash chord, (e.g. when a pedal tone is being used) and other times it makes more sense to describe the chord as what the notes add up to. The following example demonstrates both a slash chord (F/C) and a minor 7th chord created by substituting an Eb triad for a Cm7 in the right hand part.
