Chords are indicated by a chord symbol above the music notation (see below). In the case of major chords, the symbol consists only of the letter name of the chord. E.g. a C chord is indicated by the letter Cf an A chord is indicated by the letter A, a Bb chord is indicated by the letter Bb, etc. Minor chords are indicated by the letter followed by m.
Every major and minor key contains three principle chords derived from the scale.
The most common sequence of these three chords is from tonic to sub-dominant to dominant 7th and back to tonic. In the key of C major, C I is the tonic, F IV is the sub-dominant and G7 V7 is the dominant 7th. Thus the basic chord sequence is: C - F - G7 - C, as illustrated below: