The six string bass is an instrument which, as the name implies, is an electric bass with six strings. Many bassists frown on them for one reason or another, but they do provide a range one or two octaves greater than that of a four string bass. This can be handy when transcribing piano guitar or cello music. A six string bassist will sometimes stick to the middle four strings, leaving the extreme high and low for effect.
Most six strings in the mid to low price range have a muddy sound on the lowest string, which detracts somewhat from their appeal.
A there are two primary tunings with the six string bass, from bottom to top, B-E-A-D-G-B and B-E-A-D-G-C. The former mimics the tuning of a guitar, while all the strings in the second set are a fifth apart, making it somewhat easier to improvise. | 
