A surgical instrument has first and second components that can rotate relative to one another and a locking mechanism to selectively restrict or allow rotation between the first and second components. One component has a first array of two or more grooves or teeth spaced apart at a first angular pitch and at least one further groove or further tooth which does not correspond to the first angular pitch. The other component comprises a first tooth or a first groove to engage one of the first array of grooves or teeth to selectively restrict rotation to two or more predetermined rotational positions the other component also selectively engages the at least one further groove or further tooth on the first component to selectively restrict rotation between the first and second components at a 0° rotational position.
