A typical use of an engineers try-square is to mark out material for cutting/shaping.

The try square is pushed against a straight side of the material (eg. steel). An engineers scriber is then used to scratch a line onto the surface of the metal. Sometimes engineers blue (a dye/ink) is wiped onto the surface first so that the scratched line can be seen easily. The material is then cut down to this straight line.

Look closely at an engineers try-square, you should see an interesting feature. There should be a small slot that has been cut into the stock. This prevents small burrs caused by filing from altering the try-squares accuracy. Dirt can also collect on metal surfaces, again the slot helps prevent measuring angles inaccurately.
In the example seen opposite, the try-square is used to test that a 90 degree angle exists across the edge of the steel. Although a burr exists on the edge of the steel it fits into the slot and does not affect the way the try-square is used.

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