Mitre cuts are an important part of adding new trim to a room, as they allow two pieces of trim to come together in the corner of a room smoothly. Even a corner which appears to be a 90-degree angle can be slightly acute or slightly obtuse, and should be measured before cutting as even a few degrees of difference can lead to your trim not fitting together perfectly. By using a mitre gauge on each corner, you can be sure you are cutting at the right angle.
Place the mitre gauge against the corner being measured. The gauge features a circular centre, with two arms extending out from it that can be adjusted. The corner of the wall will nestle into the point where the two arms meet, and each arm will run flush against one wall.
Read the number on the dial and write it down. The dial features two sets of numbers, an inner ring and an outer ring. The inner ring tells the required angled cut for a single piece of trim to sit flush with the wall. In most instances, you will want to read the outer ring, which is for creating an angled cut to join two pieces of trim at a corner.
Use the number found in Step 2 in conjunction with the dials on your mitre saw to properly measure and cut the trim at the angle required.
Hold the straightedge horizontally, parallel to the ground, flush along one wall in the angle. A portion of the straightedge should extend out beyond the corner being measured.
Position the protractor atop the straightedge, with the flat edge of the protractor running along the second wall in the angle.
Slide the protractor so that the centre dot of the protractor, from which all the radial arms extend, is aligned over the edge of the straightedge, while keeping the flat end of the protractor flush with the wall.
Read the angle on the protractor at which the straightedge crosses the outer dial of the protractor to find the bending angle of the wall.
Divide the angle number by 2 to find the angle at which your saw must be set to make a proper cut.