Thoughts on Dovetails

Since I've been making dovetails, I've thought a lot about why they are made the way they are. The first time I tried a dovetail, I automatically laid out the joint with half tails at the top and bottom. I soon learned that this was not recommended.

Tage Frid, in the first book of his excellent series Tage Frid Teaches Wooodworking stressed that dovetails should start and end with half pins and not half tails. I believe that the reason for pinning top and bottom is to provide a mechanical restriction should either board try to cup away from the joint.

Using half pins at top and bottom theoretically fixes the pinboard and tail board from cupping. The half tail offers no such restriction. Because the angle of the tail runs with the grain rather than across it. there is nothing preventing the edges of either the pinboard or tailboard from cupping away from the joint. The first such restriction will be the first pin. So the rule of starting and ending with half pins is a good one because it improves the chances of the joint staying tight.

Today I discovered that this good-sense rule was not always practiced in the past.
If you look at the section on Antique Dovetails in this Popular Woodworking blog entry, you'll see that only one of these follows Tage Frid's admonition. Instead, most started with a half pin at the top, and ended with a half tail at the bottom. As evidenced by the half pins at the top, these craftsman obviously knew that half pins offered an advantage. But why didn't they pin the bottom? At first I guessed that they wanted the top to stay absolutely flush with the drawer because the user was most likely to notice gaps there. But if they were willing to pin the top, was it so much more work to create a half pin at the bottom?


On further thought, I suspect there was some planned advantage to this layout: there must be a reason why leaving out the bottom half pin was faster and easier. I suspect that reason is the groove holding in the bottom of the drawer. I obviously can't see the interior of any of the drawers Glenn Huey showed us, but I suspect that both the sides and front have a channel for the bottom that runs right through area of the half tail. Laying out with a half tail at the bottom allows the maker to work without fear of accidentally exposing these channels with a misplaced saw cut or chisel (something that can easily happen with a half pin). This advantage probably justifies a gamble on the stability of bottom end of drawer.