Abstract: A cutting instrument (10) comprising a handle (12) having a side-to-side thickness less than its depth in a direction transverse to its thickness and a sheath (16) movable longitudinally along the handle between a distal protective position to cover a blade (14) at a distal end of the handle and a proximal use position to expose the blade for cutting, wherein the sheath and the handle have complementary engagement formations (36,38,40) that define the protective position and the use position, the engagement formations comprising at least one engagement formation (36,38) on the handle being located on at least one edge region (26) of the handle between the sides.