What's the deal with dovetail ratios?

Lately I’ve received several questions concerning dovetail ratios. For those of you unfamiliar with them, the angle at which dovetails are cut is usually referred to by ratio, for example, 1:7 or 1:6. To draw the angle, you layout a horizontal line with a vertical line intersecting it at 90 degrees. For every individual horizontal unit, you measure 7 (or 6, etc) units vertically, with units typically being inches in today’s world.

I can never remember the actual degrees that correspond to the ratio, which is probably why we still use the ratios when talking about dovetails in today’s world. I know I could look them up, but the exact number isn’t terribly important. Wait, what? Aren’t we supposed to use 1:6 in softwood and closer to 1:8 in hardwood? Won’t Earth stop spinning if we deviate from these rules? I don’t know where this bit of conventional wisdom actually comes from, but I heard it throughout my schooling. However, when you step back and think about it, it simply can’t be true.

Hundreds of years ago, many craftsmen simply picked up a saw and started cutting, without actually laying anything out. Some still do this today (unlike me who belly aches over dovetail layout and draws 20 different variations to try to come up with something original and aesthetically pleasing – oh, the time I’ve wasted!). Then there’s the ease at which you can change the angle as you’re paring with your chisel, regardless of what you laid out on the board. Also, if you spend any time looking at antique drawers, as one does, you will notice the angles are all over the place, sometimes on the same tail or pin! This all gets further complicated by the fact that often drawers had a hardwood front and the rest was softwood. What is one to do in these situations?

Dovetails on a drawer from 1780. The slopes on each side of the bottom pin are quite different from one another.

Drawer from 1790. Slopes on tails vary from top of drawer to bottom.

I really think your choice of ratio comes down to what you find pleasing to your eye. Of course, cutting a joint with integrity that is well fit is the most crucial part to the joint standing the test of time (and abuse by the owner of your furniture). You don’t want your angle so steep as to create so much short grain that the tails break during assembly; too straight of an angle and you lose the mechanical advantage dovetails offer. Over time you will probably notice an affinity to a certain ratio; personally, I like 1:7 best. Next time you cut a set of dovetails, don’t overthink things. Layout what you think looks good and go to it. Now, if I could only take my own advice…