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A prison commissary or canteen is a store within a correctional facility, from which inmates may purchase products such as hygiene items, snacks, writing instruments, etc. Typically inmates are not allowed to possess cash; instead, they make purchases through an account with funds from money contributed by friends, family members, etc., or earned as wages. Typically, prisons set a maximum limit of funds that can be spent by each inmate on commissary.
Items used as currency
Certain items tend to be used as currency. Cigarettes were a classic medium of exchange, but in the wake of prison tobacco bans, a number of other prison commissary items have taken precedence. These include postage stamps and instant ramen noodles, which is also increasingly popular as a medium of exchange due to its versatility in prisons as food and its relative abundance. In some prisons, Mackerel has also taken prominence as a currency, as it is priced closely with one US Dollar, and maintains stability by virtue of being rarely consumed. 
Some prison commissaries are staffed by government employees and inmates, while others have been completely privatized. Significant price markups are common in prison commissaries, although some prison systems set maximum markups; for instance, the Delaware Department of Correction has a 20% maximum markup. $100 million in purchases were made from Texas' prison system alone in 2009. Prison commissary is a privilege that is often taken away for infractions.
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