A square protractor having graduations on two adjacent edges, with the centre at one corner, and ruled with a square grid having intervals of one centimetre or other linear unit. It has a movable arm turning with the centre as pivot and graduated in the same units and at the same scale as the ruled grid. In using the protractor, the arm is set to a given angle, azimuth or bearing, and the length of a line is marked on the arm; inspection of the grid at this point will give the components of that length along the axes marked by the edges of the protractor.