Abstract: An improved T-square which is self-supporting consists of a main body and a carriage. The main body consists of an elongated straight edge member having a head at one end thereof formed with a mounting channel for receiving a marginal edge portion of a drawing board for movement therealong. The carriage has a second mounting channel formed therein for engaging a support rail which is spaced from the first edge of the drawing board. The carriage is movable along the support rail. A resilient tensioning device is provided which connects the carriage to the main body. A luminaire chamber is formed in the main body in close proximity to the straight edge and a luminaire is operatively mounted in the luminaire chamber to illuminate the straight edge of the body. In addition, a sensor is provided for detecting improper alignment of the T-square with respect to the board on which it is mounted.