Lapped Dovetail

Prepare both components square and true. In this example, both are the same width, but they need not be.
Decide how thick a lap you desire. 1/8" (3mm) is a sensible minimum, to avoid ghosting through of the tails on the lap.
Use a marking gauge to gauge the lap line from the inside surface of the pin board,
before using the same setting to gauge all around the end of the tail board.
For drawers, mark in the position of the groove for the bottom. We want to ensure the groove is run through a tail and not a pin.
A groove cut in the tail will be hidden from the outside once the joint is assembled.
Use a dovetail marker to align the outside edge of the first tail, capturing the bottom groove in the case of a draper side.
Now mark the outside edge of the last tail, and use dividers or a ruler to space out the desired number of tails in between.
Here I have marked out for two tails, with a slim pin separating them.
Next, transfer the edges of the tails across the end of the tail board with a try-square.
These lines define the cuts we need to saw for the tails. You might like to hatch the waste now, before sawing.
Line the saw up with the end marks, and angle it as per the face marks, and saw down to the gauge line. Don't alter the angle during the cut.
Rotate tail board and saw the outside shoulders of the outside tails, up to the gauge lines.
Chop out the waste between the tails. Start ahead of the gauge line and work back towards it, to avoid 'pushing' the line back due to the chisel's bevel. Remove to just over half depth from one face of the board, before flipping over to complete.

Put the tail board back in the vise and clean the shoulders down to the gauge lines.

Both shoulders and sides of tails should be flat surfaces.

The sides of the tails should also be perpendicular to the face of the board.
Hold the pin board in the vice and lay the tail board in position on top of it.
The shoulders should line up with the face side.
With the tail board held firmly in position, carefully mark the tails onto the pin board.
Make sure the edge of the blade stays tight to the tail, but doesn't cut into it.

Gauge the width of the tail board on the inside face of the pin board.
Then use a try-square to extend the pin lines onto the inside face, down to the gauge line.
Saw the pins by following the knifed lines, keeping to the waste side, and stopping at the two gauge lines (on the end and on the inside face).
You are aiming for something that looks like this:
With the pin board clamped to the bench, you can now chop the waste out from between the pins. Work back from the end, towards the gauge line.

To clear into the corners, use a pair of skew chisels, your narrowest bevel edge chisel, or a sharp straight bladed knife, referencing off the gauge line,
and by paring in line with the knife lines with an ordinary bevel edge chisel.
If you marked the pins accurately from the tails, and have cleared all the waste back to these knife lines, then the tail board should fit snugly into the pin board.
Apply glue to all the long grain joint surfaces, and assemble the joint. You may need to tap it home, using a block to protect the tail board. No clamping should be necessary, but just check that the joint is at a prefect right angle. You should be able to adjust to ninety degrees very easily by hand, and the joint should happilly remain there as it dries.

Now watch the video:

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