How do prisoners trade currency?

In prison, inmates often trade items that implicitly act as currency, since actual cash transactions are typically prohibited. For example, packages of fish (such as mackerel), which are disliked by the prisoners but used as a form of currency, are common. A pack of mackerel might represent a certain amount, like $1. Another common form of currency is stamps, where a book of stamps might be equivalent to $10. These items are traded for goods and services within the prison, and sometimes they are used in a process where an inmate has a family member or friend on the outside who converts these traded items into real money through services like Western Union 1.