12 Bar Blues is a pattern of chords which repeat every 12 bars. There are hundreds of well known songs based on this chord progression, i.e., they contain basically the same chords in the same order. 12 Bar Blues is commonly used in Rock music and is the most common progression in Blues music, as well as occurring frequently in Funk, Soul and Jazz.
The following example is a typical example of a 12 Bar Blues bass line. It is played as a shuffle, which means a constant stream of swing eighth notes. Shuffles are very common in both Blues and Rock. It is common to play most of the notes on the beat staccato even though it is not notated. The expressions used in a bass line are often left up to the individual player.