Bottle cap

Bottle Caps are the standard currency in the first Fallout game. In Fallout 2 regular money has been introduced and bottlecaps have been rendered useless and can only be found in Broken Hills. Bottle caps make a return as the standard currency of the Capital Wasteland in Fallout 3. In addition to bottlecaps, a form of pre-war currency (most likely American Dollars) can also be found in the ruins of DC which can then be sold to merchants in return for bottle caps. Bottle Caps can also be put to use in weapon form as one of the major components of a Bottlecap Mine.

Bottle caps are backed by The Hub merchants. The reasoning for their support is that the technology to manufacture bottle caps and paint the surface has been lost in the Great War, which would greatly limit any counterfeiting efforts. Secondly, there is a limited number of bottle caps, which would serve to preserve their value to some degree.

The issue of some concern is that with time, number of bottle caps will decrease due to wear.

Hypothesis of Depiction
The graphics show bottle caps as having the rimmed edges pointing outward. This depiction is clearly a production decision to communicate the "bottlecapness" to the player; in real life the caps would have likely ended up flattened over the passage of time, or intentionally pressed to increase the carrying capacity for the individual.

Kapsle