Bench dog
Also called: dog, holdfast
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A bench dog is a stop on a workbench that a board can be pressed against while it is being worked.
Using the stop
Use the dog as a reference for joint layout and positioning. Its resistance to sideways movement lets you set a shoulder, the step at the end of a joint, accurately before fixing the parts.
Do not confuse it with a holdfast
A holdfast is a different bench aid: an iron device that grips work firmly against the bench. A bench dog provides a stopping point, whereas a holdfast presses down and secures the work.