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Use a chisel that is the same size as the mortise. For a 3/4" mortise, use a 3/4" chisel.
A mortising chisel has straight, vertical edges. It drives in and stays on the same line. The beveled-edge chisel is an all-purpose chisel used for many applications in the shop. It can be turned in a cut.
Tip: Make sure the chisel is sharp before using. The bevel and the flat back surface need to be sharpened.
When starting the cut, keep the back surface of the chisel to the inside of the mortise. If the beveled surface is pointing into the mortise, the beveled edge will push against the wood and can alter the line of the cut.
Mark out the outline of the mortise all the way around with the beveled edge pointing away from the cut.
Wedge the material out of the mortise with the beveled edge of the chisel and a hammer. You can use a metal hammer with a metal chisel, but don't use a metal hammer with a wooden-handled chisel. Use a rubber mallet instead.
Use a smaller chisel to pare out the extra material.