The bass guitar is founded on electric guitar technology, so its not surprising that its strings share the same basic materials - nickel-alloy, nickel-steel and stainless steel. As with the guitar, roundwound strings are generally favoured by rock players, while jazz players often use flatwound strings, particularly on fretless basses.
Again, sets vary a little between manufacturers, but the chart below gives a general picture:
4-string | 5-string | ||||
G | D | A | E | B | |
Extra-Light | .040 | .055 | .075 | .095 | .128 |
Light | .040 | .060 | .080 | .100 | .128 |
Medium Light | .045 | .065 | .085 | .105 | .130 |
Medium | .048 | .067 | .085 | .106 | .130 |
Heavy | .050 | .070 | .090 | .110 | .135 |
As with all strings, theres a trade-off between tone/volume and playability - heavier strings sound better but can be harder to play, especially for beginners.