Prison slang is an argot used primarily by criminals and detainees in correctional institutions. It is a form of anti-language. Many of the terms deal with criminal behavior, incarcerated life, legal cases, street life, and different types of inmates. Prison slang varies depending on institution, region, and country. Prison slang can be found in other written forms such as diaries, letters, tattoos, ballads, songs, and poems. Prison slang has existed as long as there have been crime and prisons; in Charles Dickens' time it was known as "thieves' cant". Words from prison slang often eventually migrate into common usage, such as "snitch", "ducking", and "narc". Terms can also lose meaning or become obsolete such as "slammer" and "bull-derm."