The underground economy in prison is way more sophisticated than you might think.
Inmates aren’t allowed to carry cash. What they earn from prison jobs goes into commissary accounts, which allows prisoners to buy better food and other personal care items from the prison store.
There are other currencies, though. One of the most popular items is mackerel– or “mack” in prison lingo – which costs about $1.47 a can at the commissary, according to Michael Smith Baker, an inmate at New York Department of Correction.