The most common chord structure uses three chords – the tonic (chord I), the subdominant (chord IV) and the dominant (chord V). In the key of C this would be:
|Phrase 1||C (tonic)||C (tonic)||C (tonic)||C7 (tonic)|
|Phrase 2||F (subdominant)||F (subdominant)||C (tonic)||C (tonic)|
|Phrase 3||G (dominant)||F (subdominant)||C (tonic)||G (dominant) to repeat or C (tonic) to finish|
There are many variations on the above structure. Sometimes 7th chords are used (the same chords with an added 7th). The 12-bar blues form the basis of R&B (rhythm and blues), rock and roll and jazz music.
Compared with the major scale, some notes, known as blue notes may be flattened by a semitone or ‘bent’ by a smaller interval. Blue notes are usually found on the third, fifth or seventh degree of the scale.
Guitarists can ‘bend’ the notes out of tune by pushing the strings sideways. A bottleneck is sometimes used to slide up and down the guitar strings. Both effects sound like a vocal cry.
There is bottleneck guitar in the song Travelling Riverside Blues by Robert Johnson.