A composite trial prosthesis comprises a first component (2) and a shim (4). The first component (2) has a first surface (8) corresponding to a surface of a prosthetic component and a reverse surface (10). The shim (4) is arranged to couple to the reverse surface (10). The first component (2) further comprises a first marking (24) which is arranged to be visible when the first component (2) is coupled directly to a bone or a further prosthetic component. Part of the shim (26) is arranged to obscure the first marking (24) when the shim (4) is coupled to the reverse surface (10), the shim (4) further comprising a second marking (28) which is visible when the shim (4) is coupled to a bone or a further prosthetic component. A set of composite trial prostheses and a method of selecting a required surgical prosthesis are also disclosed.