In some musical situations, it sounds good to sustain one chord while the bass plays a line underneath it, as demonstrated in the following example. The chord symbols change every two beats, but the guitar part remains a C triad.
The changing symbols indicate the moving bass part which descends through the C major scale under this triad. Try this process on the guitar and listen to each new bass note changes the effect of the chord.
This technique can be used with any type of chord in both major and minor keys. In the following example, a Cm triad is played over a bass line which descends through the key of C minor. Both the natural and flattened 7th degrees are used in the bass line.