Post-War currency

The currency in the Fallout world has varied from game to game.

In Fallout 1 bottlecaps were used as currency.

In Fallout 2 it was dollars backed by gold in value and by the 3 big powers of New California Republic, Vault City, and New Reno, with bottlecaps becoming little more than nostalgia as at least ONE of the 3 described powers had influence in just about any civilized spot in the Wasteland.

In Tactics, you used a mixture of Brotherhood scripts and Ring pulls, essentially the same as bottle caps only also including pull-tabs from soda cans.

For Brotherhood of Steel, well, no one really cares, do they? (its Caps, though due to blatant product placement replacing beloved in-universe brand Nuka-Cola, BAWLS bottle caps are worth extra)

For Fallout 3, Bottle caps return as the main form of currency, most likely due to the Capital Wasteland's isolation from the more civilized western coast. Interestingly enough, one can also find pre-war money though it is simply nothing more than weightless loot that fetches a nice price. Also of note is that unlike previous Fallout games, when one drinks a Nuka-Cola, its bottlecap actually adds to your stock.

Waluta