A surgical instrument for attachment to a cutting block having a cutting slot defining a cutting plane is described. The surgical instrument comprises a dual purpose indicator (2) member. The indicator member is configured to (i) allow visualization of a first plane and (ii) to indicate a predetermined distance from a second plane perpendicular to the first plane. The surgical instrument also comprises an attachment member (14,16,18,20) for attaching the surgical instrument to the cutting slot in either of two mutually perpendicular orientations such that either the first plane or the second plane is parallel to the cutting plane. The surgical instrument can therefore be used both to visualize a plane of a cut and to indicate a distance from the plane of a cut, depending on the orientation of the attachment member in the cutting slot. This allows inventory to be reduced because the surgical instrument can replace two prior art instruments. The surgical instrument may form part of a system with one or more cutting blocks.