There are three main types of chords, namely, majors, minors and sevenths. So far you have had examples of majors and minors. Here are three seventh chords: -
A7
D7
E7
A7
An optional fingering for the A7 chord is shown here. The first finger bars 3 strings.
The use of the seventh chords as substitutions for major chords can make the 12 bar blues progression more interesting:
Rhythm Pattern
You will notice in bars 9 to12, that there are two chords per bar. Each chord is given two counts. Once the suggested rhythm pattern has been mastered, try some other variations (more difficult) of down and down/up strumming.
* The correct name for these chords is "Dominant Seventh", however the name is often abbreviated to "Seventh" and written as such.
In any turnaround a seventh chord may be played in the last bar, in place of the major chord; e.g., Turnaround No.1 in the key of G: -
Rhythm Pattern
This seventh chord (D7) will "resolve" (i.e., lead back) to the first chord (in this case, G).