Abstract: An improved square for use on outside corners formed by approximately right angularly arranged first and second surfaces and an outwardly projecting obstruction at the corner. The square comprises first and second right angularly arranged arms each with a precise inwardly facing linear edge surface. Each of the arms terminates a substantial distance from an imaginary inside corner residing at the intersection of first and second lines projected respectively from said inwardly facing linear edge surfaces on the first and second arms. An interconnecting means between the first and second arms takes a generally particircular form with opposite ends thereof connected respectively with the arms and with an intermediate portion extending arcuately outwardly and defining a clearance opening in the general shape of a segment of a circle.