An engineer's square gives you four 90° angles: internal, external, outside blade to inside stock and inside blade to outside stock. Compare this to a carpenter's try square which only guarantees accuracy on the internal angle. Our engineer's squares comprise a stainless steel blade, triple riveted into a steel stock. The hardened and tempered blade and the high quality steel stock are ground and polished for accuracy. Final polishing makes the rivets virtually invisible. All Axminster Precision Engineer's squares conform to DIN875/II, guaranteed square and straight. The size stated is from the inner edge of the stock to the tip of the blade. The small notch on the inside corner of the square prevents accumulations of dust and dirt, small burrs or saw whiskers on the workpiece affecting the accuracy of the reading. Supplied in a foam lined case. N.B The blade measurements given are the inner diameter of the blade.