Abstract: A trim installation square device is provided for facilitating measurement, cutting and installation of trim around a door or window opening. The device includes a base member, and extension member extendably coupled to the base member, and a locking assembly for holding the base member and extension member in place. The base member and the extension member are positioned relative to each other such that a base member first edge is parallel to an extension member first edge and a base member second edge is parallel to an extension member second edge forming two spaced right angles. A ruler is coupled to one of the base member and the extension member. The ruler is positioned for measuring a transverse distance between the base member first edge and the extension member first edge.