Read how to cut them, or watch the video
The first step in cutting a through dovetail, is to prepare the two pieces true and square, and to ensure the ends to be jointed are square to their sides and faces (as right)
The edges of what will become the 'pin' board (the dark wood in this example) need not be marked.
These light gauge lines can be planed off once the joint has been assembled.
A dovetail marking gauge, or a bevel gauge, can now be used to mark out the sloping saw cuts (as left). Either pins, or tails (as in this example), are marked.
Once the slopes have been marked, perpendicular lines are marked, across the end grain for tails, or to the gauge line for pins.
The waste to be removed is indicated with a cross or hatching (as left), since it is very easy to end up cutting away the wrong bits.
With a dovetail saw, the tail sides, and the baseline of the outside tails can be cut, keeping the saw kerf to the waste side of the lines (as left).
When sawing the sides of the tails (or pins, if you start with these) the most important point is to saw in a single plane. Eye up the saw with the sloped line, and perpendicular to the face of the board for tails, or end of the board for pins.
The bulk of the waste between the tails (or pins) can be removed by sawing on the waste side of the baseline with a jeweler's saw or a coping saw.
Now remove any remaining waste with a sharp chisel, being careful to maintain flat sides to the tails, and to not cross the gauged lines.
Position the tail board on top of the end of the pin board, aligning the gauge line with the inside corner, and mark the tail slopes onto the pin board using a knife.
(If you cut the pins first, position the end of the pin board on to the face of the tail board, aligning the inside corner with the gauge line of the tail board.)
Now using a try-square, mark perpendicular lines down from the knife marks, to define the sides of the pins.
(For a tail board, mark perpendicular lines across the end of the board.)
Saw the pins (or tails) on the waste side of the marks. Sawing right to the line, but not crossing it is the aim.
Once again, use a jeweler's saw or coping saw to remove the bulk of the waste.
Then remove any remaing waste with sharp chisels. Final paring should be done with the chisel tip registered in the knife line.
The joint should be a snug fit, but not require a mallet or clamps to close it up. If it is too tight, check that you have pared away all the waste.
Once the fit is good, you can glue up. Apply a thin coat of glue to the joint surfaces of the pin board, and assemble the joint. Check, and adjust if necessary for square, using a try-square, and leave to cure.
Once the glue is cured, take a few clean-up shavings with a plane to true and level the joint.
Practice makes perfect, so if you are not happy with the result, saw the joint off and have another go.