Accompaniment
Accompaniment is the chords that go with a melody. On the piano the right hand plays the melody on the treble staff and the left hand the chords on the bass staff.
Which chord fits a measure? Choose the chord that contains most of its notes. A measure of C, D, E, C: C and E are in the chord C, D only passes between them – so C. A measure of F, G, A: F and A are in the chord F. The chord letter above the measure tells the left hand what to play. If a chord clashes with the melody, try another.
The simplest way: the left hand plays the chord on beat one, the strong beat, and holds it for the whole measure. Livelier: the chord on “one” and “three”, as two half notes. Two more ways from lead sheets: the root on “one”, the whole chord on “three”; or just the root and the fifth (C: C and G).
The left hand plays the chords low, below middle C, and in inversions so it moves less: C–E–G, C–F–A, B–D–G. Three chords carry hundreds of songs.
What it sounds like
Melody above and chords below blend into one sound: the chord gives support, the melody movement. Swap in a wrong chord and you hear it rub against the melody at once.
Where it is taught: the studio’s interactive textbook, chapter 10, “Accompaniment: melody right, chords left” and chapter 19, “Lead sheets: melody and chord letters”.