How are basses tuned?

Four-string electric and upright basses are tuned in perfect fourths, E A D G from low oo high. Five-string basses commonly add a low B, but some add a high C instead of the low B. Six-string basses usually have both the high C and the low B. Seven-string basses add an F# to the six-string layout. Eight-string basses are like 12-string guitars. There's an extra string added to each of the four standard strings, with the second set tuned an octave higher. 12-string-basses add two "octave" strings instead of one, making 4 sets of three strings.