Bass guitar tuning
Most bass guitars have four strings, which are tuned one octave lower than the lowest pitched four strings of an electric guitar. Thus, the bass guitar's standard tuning is E, A, D, G (lowest to highest string.) 
|4 String||5 String||5 String
A popular altered tuning is Drop D, which decreases the E string by a whole step downward. This provides a low D, which is desirable for songs in the key of D or G.