The implement denoted by Fig. 289 is a tee-square, and consists of a pedestal having a blade at right angles to the pedestal, but situated midway from each end instead of at one extremity, as in an el-square. Such a tool is sometimes employed for work that requires a square to be applied to the outer surfaces, but a tee-square cannot be applied to an inner corner; the tool is therefore specially adapted to measure and discover conditions of' holes in levers, wheels, bosses, and other work having openings through or partly through. Tee-squares are useful while forming rectangular holes, and for marking the places of intended key-ways in circular holes; also for marking slots and key-ways in ends of bolts, rods, and axles. The tee-square shown by Fig. 290 is a bisector or centre-finder. The instrument consists of a tee-square whose pedestal is furnished with two projecting pins so adjusted that a straight line connecting their centres is at right angles to the length of the blade ; both pins are also equally distant from the blade, so that the centre length of the blade is exactly midway between the centres of the two pins. A square of this sort is applied to use by placing its blade across an end of a rod, bar, or shaft that requires its centre to be found; and at the time the blade is put across the two pins are put in contact with one side of the work ; while in this position two lines are marked with a scriber across the end of the work, one line being at each edge of the blade; after two lines are thus scribed upon the shaft-end, the centre line required is marked midway between the two by means of a divider and straight-edge. When one centre line is found by such means, the square may be again applied to the work so that the blade shall be at right angles to its former position; and while it is thus held two other lines are scribed across the two first made, and the centre discovered as before. Bi-secting by means of such an instrument may be further simplified by so placing the two pins that one edge of the blade shall be midway between the two pin-centres, instead of the centre length of the blade being in this situation; by this arrangement only one line is marked instead of three, this one line being the centre line required.