Abstract: A case knife (1, 101), generally consists of a pair of substantially symmetrical handle shells (13, 112). A single edge razor blade B is removably supported in one end of the handle formed by these handle shells. Each handle shell includes two moving parts—a guard (10, 110) and a thumb or finger operable button (5, 105). Each such guard, when paired with a substantially identical guard of the other handle half shell, flanks the otherwise protruding sharp edge of the razor blade. Each guard can move independently to expose a protruding corner of the razor blade. The operable button is mounted on an axle or lug (14, 103) that passes from the inside of a handle shell, through to its outside surface. This axle is in turn fixed to a cam follower (20, 120) mounted on the inside of each shell.