The letters above the staff are chord symbols. Chords are groups of notes played by the rhythm guitarist or keyboard player.
The three most common chord types are major, minor (m) and seventh (7) chords. Chords will be discussed in more detail later in the book.
The following example uses the 12 bar progression. The bass plays root notes only for each chord. A root note is the note with the same name as the chord and will always blend well with that chord.