Massachusetts Jails are for individuals who are held awaiting trial or who are Serving Sentences of two and one-half years or less.  Prisons are for people serving sentences that are longer than two and one-half years.  District courts may only sentence convicted individuals to jail.

Historically, counties ran the jails and the Commonwealth ran the prisons.  This is no longer always so, because several Massachusetts county governments no longer exist.  The Commonwealth now performs their functions, including jail keeping.  However, each county still elects its own sheriff to be its jail’s chief officer.