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.