A squareness testing instrument (1, 100, 200) has one or more pairs of arms (2,3, 122,123), such as a one or more pairs of slats or rods, pivotally interconnected using suitable fasteners (4). The one or more pairs of arms are shaped such that they can fit substantially squarely into a corresponding imaginary rectangle (6A, 6B, 106, 206) with at least some arm ends (8, 108, 128, 208) in contact with sides thereof irrespective of the relative pivotal positioning of the arms. The instrument is positionable inside a frame type structure (14, 114) and pivotable apart to try and bring at least some of the arm ends in contact with sides (18, 19, 118, 119) of the structure to test at least one of the squareness and parallelism thereof.