Abstract: A T-square includes a first member defining a straight edge, a second member that is mounted transverse to the first member and defining a free end spaced from the straight edge, and sheet material securing means mounted to and extending beyond that free end. In various embodiments, the securing means holds sheet material in compression against the straight edge of the first member or in compression against an undersurface of the second planar member, or both. The securing means may be a clamp that is moved toward the straight edge by a threaded shaft, or it may be a biasing member such as spring steel that holds sheet material in compression against the straight edge or against both the straight edge and against the second planar member.