There are a number of tools available to accurately mark guidelines for cutting etc. These are called squares, bevels and gauges.
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A gauge is a simple but effective carpenter's tool for accurately scoring guidelines on timber.
A marking gauge consists of two main parts: the stock and the stem. The stem is inserted through a hole in the stock just slightly larger than itself. This allows the stock to move up and down the stem. A retaining nut on the side of the stock is tightened to fix the stock firmly in position. A marking pin is positioned towards the top of the stem.
The distance between the marking pin and the top face of the stock will be the measurement scored into the timber. Set the gauge at the required measurement, tightening the retaining nut to hold the stock firmly in position. With the top face of the stock pressed against the edge of the timber and the marking pin against the surface of the timber, draw the gauge along the whole length of the timber. The marking pin will leave a scored guideline on the timber parallel to its edge.