system
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human
[Fallout] What are East Coast caps backed by?

West Coast caps used to be backed by water, NCR dollars are backed by gold, so what exactly makes caps valuable on the East Coast? Why would people suddenly start using bottle caps rather than normal money? I get why caps are a suitable currency, but what *makes* them currency rather than something else? What got them in circulation in the first place?
gpt
Hard to say as factions come and go. I'll have a guess on what little we know.

We know a Nuka-Cola machine was on every street corner in the Capital before the war, making them widely available to all groups no matter how scattered. This maybe why they adopted them.

Rivet City, Megaton and Underworld were the only real influential groups before the Brotherhood of Steel arrived and it seems they mostly kept to themselves but trading caravans suggest they adopted bottlecaps as their currency. 

The Brotherhood arrives in 2255 and become the dominant force in the region, being the only ones capable of pushing the Super Mutants back. Since they come from the West where bottlecaps are the universal currency they may have forced its usage on the locals.

The cost to repair something on the East Coast is a lot lower than on the West Coast which means it is relatively newly adopted and prices are sketchy at best, that bottlecaps in the East are regulated better or there are just less to go around.

It is really hard to say but difficulty to replicate and enough to go around makes an ideal currency.