Abstract: A device for measuring ‘square’ of a building structure having walls, floors, and ceilings. The device includes a substantially longitudinal planar body having a mounting portion for receiving a handle member. The handle member is adapted to be adjustably mounted on or adjacent the mounting portion of the body to allow selective positioning of the handle member relative to the body. The mounting portion of the body includes a releasable locking means which locates the handle member substantially perpendicular to the body in an operating condition for performing square measurements of floors, walls, and ceilings of the building structure or the like. In a released condition, the locking means allows the handle member to fold against the body. The device can further include a marker device mounted on a portion of the longitudinal body with a trigger mechanism which initiates release of marking fluid when the device indicates off-square.