Dovetail Errors of the Past

In my basement shop I have my first "successful" project in wood. I put that in quotes because the criteria for success were pretty loose: could I make dovetails that actually seemed to stay together by themselves? I did pretty well for such a complex first project, but you can see the single nail in one of the pins that says "not quite"

There were a number of mistakes that preceded the making of the dovetails, (like trying to smooth wood scavenged from a pallet without first checking for nails, not making the boards flat and square, and not actually knowing how to sharpen a plane blade OR a chisel very well, but that's all for another day). But I was making dovetails, and I was not daunted by a lack of knowledge. I had read instructions for making dovetails several times before, so I thought I had the right idea: mark out the pins and cut them, mark the tails from the pins and cut them, put together the box and wonder how you ever became so clever. So I did just that and was very satisfied.

Of course there were a number of execution errors (obvious in these pictures), but there was a major layout error too. One I've seen other woodworkers duplicate (including one whose project was featured in a major woodworking magazine): I'd created half tails instead of half pins. I think anyone layout out dovetails for the first time is likely to do this if they don't have guidance: It just seems logical to put those satisfying visible angles in as many places as possible. But the experts all tell you to use half tails on the ends of a join, and after much puzzling about the joint structure and wood movement, I think I know several reasons why.

1. If the wood cups,half tails allow it to happen much more easily
2. There is less distribution of pressure across the grain structure, making it more likely for joint failure at the corners
3. If you are making a half blind joint to allow a panel bottom, you need to place the panel groove much higher in the frame, losing some of your drawer space to the underside.

There may be other reasons, but these are the three I've encountered so far. Next time you look at a well made dovetail, take a look at those half tails and try to picture the forces on them. You'll begin to see how having these cap the joint makes it much more difficult for cupping to occur in either panel that if it had the straight on angle of a half tail.