Although the terms “jail” and “prison” are commonly used interchangeably, there are important differences. Jails are locally run facilities that primarily hold unconvicted people who have been arrested and are awaiting a resolution to their case. Many people are in jail simply because they cannot afford to pay bail, fines, or fees. Other people are in jail serving shorter sentences for misdemeanor or low-level felony convictions, or are locked up for violating probation or other forms of supervision. In some states, jails also hold people serving state prison sentences, as well as people being held for federal authorities. Prisons are state or federal institutions where people who have been convicted of crimes are sent to serve sentences of imprisonment. Since around one in three incarcerated people nationwide are held in jails, incarceration must be measured using numbers that represent people in both jails and prisons.