An eight bar blues is a typical blues chord progression, taking eight 4/4 bars to the verse. Heartbreak Hotel, How Long Blues, Trouble in Mind, Ain't Nobody's Business and Cherry Red are all eight-bar blues standards. One variant using this progression is to couple one eight-bar blues melody with a different eight-bar blues bridge to create a blues variant of the standard 32-bar song. Walking By Myself, I Want a Little Girl and (Romancing) In The Dark are examples of this form.
Eight bar blues progressions have more variations than the more rigidly defined twelve bar format. The move to the IV chord usually happens at bar 3 (as opposed to 5 in twelve bar.)
Worried Life Blues (probably the most common eight bar blues progression):
I I IV IV
I V I-IV I-V
Key to the Highway (variation with the V at bar 2):
I V IV IV
I V I V
Walking By Myself (somewhat unorthodox example of the form):
I I I I
IV V I V
(The same chord progression can also be called a sixteen bar blues, if each symbol above is taken to be a half note in 2/2 or 4/4 time -- blues has not traditionally been associated with notation, so its form becomes a bit slippery when written down.) Ray Charles's original instrumental Sweet Sixteen Bars is an example. (Courtesy of Wikipedia)
The 8-bar blues progression is similar to the much more popular 12-bar traditional blues progression except that bars 3, 4, 8 and 10 are eliminated in order to shorten the sequence. The turnaround continues to be comprised of the last two bars of the progression with many possible substitutions available. Below is an example of a proto typical progression in the key of C followed by several common variations.