1. A device for cutting a predetermined thickness of a piece of bone, comprising: a support block which in use is fixed relative to the bone, the support block has a plurality of recesses adapted to receive a stopper; and a guide block having a wedge defining an opening for receiving a cutter to guide cuts, the guide block and support block being arranged to permit relative movement there-between, the guide block includes a plurality of holes, only one of which holes is alignable with an associated recess on the support block, so that a stopper is able to pass through the hole into the recess at a unique configuration of the guide block and support block where the hole aligns with the associated recess, thereby arresting relative movement between the blocks, such that in use at a first predetermined configuration of the blocks, the cutter makes a first cut, and at a second configuration of the blocks the cutter makes a second cut, the two cuts being parallel one to another so as to define the predetermined thickness of the piece of bone.