A surgical instrument, for example a cutting block for use in knee surgery, comprises a mounting surface defining at least one opening for receiving a mounting projection. The mounting surface further defines a blind recess which extends linearly from the at least one opening. The blind recess may extend to an edge of the mounting surface. A system including a surgical instrument and one or more mounting projections is also disclosed. The blind recess can be used as a track to guide a projection to an opening, increasing the area of the instrument that can be used to locate the projection in the correct position. The recess acts as a guide for the projection to enter the opening by providing tactile feedback to guide the projection towards the opening. If the recess also extends to the edge of the instrument, the recess may be visible more easily when the surgeon is installing the instrument providing visual as well as tactile feedback.
