How are basses tuned?

Four-string electric and upright basses are tuned in perfect fourths, E-A-D-G from low to high. Five-string basses most often add a low B, but some add a high C instead of the low B. Six-string basses have both the high C and the low B. Seven-string basses add a high 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. There are even 12-string-basses (although quite rare), which add two "octave" strings, making four sets of three strings.