When fitting a dovetail joint I cut the tails first, align the tail-board over the pin-board, and transfer the tails with a marking knife. Pretty straightforward, but it can be tricky keeping the joint aligned while transferring the tails to the pin-board. If the tail-board is cooked the drawer/case/etc will be racked when assembled, there will also be gaps at the joint’s baseline. If the tail-board is square, but slides a little forward or back relative to the baseline the joint will have gaps or may not assemble at all. Finally, if the board is perfectly aligned to begin with but then shifts while marking… good luck realigning.
For all these reasons, I put a small rabbet on the tail-board which creates a barely visible shoulder. This shoulder slides against the pin-board and instantly aligns everything except the sides of the boards, but that’s simple to do. If you don’t rabbet your boards already, you should. At least give it a shot. Once you decide to use the rabbet you have to figure out how to make it. The generic answer for a large number of joints is to set up a table saw or router. That works great, but for working on a few joints (making a box or just a couple drawers) it’s faster by hand. Since you’re cutting a rabbet, it makes sense to reach for a rabbet plane or fillister. After all, this technique is known as “the 140 trick” named after the model #140 block plane often used to cut the rabbet. These planes work well for larger cases or drawers, but I find them completely unsuited when working with narrow parts. The planes can be sensitive to set up and a narrow board doesn’t give much surface area for the plane to rest on or the fence to register against. Clamping small pieces securely for use with a rabbet plane can also be a challenge.
Enter the router plane. I typically have two boards (a left and right side) to rabbet, so I lay them next to each other on the bench, throw a clamp on the back side and let the front register against a bench dog or some other backstop. Using the router plane, I support half of the tool on each board (which should be the same thickness), set the depth with extreme simplicity and accuracy, and cut across the grain to my baseline. The baseline is already cut into the board with a marking gauge as part of the dovetailing process so no extra work there, but you do have to be careful not to cut across the baseline. This may sound difficult, but I’ve never had an issue. The knifeline from the marking gauge severs the fibers should cut deep enough that as the router plane gets close the fibers simply lift up and fall off. When all is said and done I find this approach far superior for narrow components. Less tool set up time, no risk of tipping the tool resulting in a non-square rabbet, and work holding is much easier. If that’s doesn’t convince you, check out the video of the technique in action, then give it a try.