A mandolin is tuned in fifths like a violin, but in courses of two for a total of eight strings. The pairs are generally tuned in unison. From the top down:
E, A, D, G
This is standard tuning. Bill Monroe (the Father of Bluegrass) would often "cross-tune" his mandolin to different notes, much like an open tuning on a guitar to get kind of a drone effect. Another great contemporary player, Radim Zenkl, often tunes his mandolins into all kinds of interesting combinations, with fascinating results.
