I teach a hand cut dovetails class and try to stress to the students the important things to do and check when making hand cut dovetails.

I teach doing tails first. After they have cut the tails and cleaned out the waste, I show them how to check the area before transferring the tails to the pin board. For one check, I use a double square.

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It can be used to check the area where the waste was removed to make sure that it does not have a "belly" that will push the pin board away from the tail board when they are put together.

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A problem most beginners have is sawing straight across the board. So sometimes a tail is wider on one side of the board than on the other side of the board. You can try to check for this with a double square but you can only check the end of the tail.

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If the tails are not square cut, when you transfer the tails to the pin board, and cut the pins, you'll wind up with too tight a fit, or too loose a fit.

What I found recently is a replacement "blade" for a double square from Sterling Tool Works. Here's the blade in a double square.

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The small probe can be used to check along the sides of the tails, even with very small pins.

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I used it in class this past weekend and it helped the students get better fitting dovetails.

I have no affiliation with Sterling except that I bought one of their blades.

Mike