Dovetail joints are among the most aesthetically pleasing joints, and if correctly constructed they can also be the strongest. The locking of the pins into the tails provides a very strong mechanical joint that has many advantages. Dovetail joints also allow for the expansion and contraction of the wood, without compromizing its structural integrity1. When joining large expanses of wood, such as for case sides, this is extremely desirable. The dovetail joint also allows woodworkers to create projects that are made entirely of wood, with no hardware visible.
The strength of the dovetail joint comes from two things: the mechanical interlocking of the pins and grooves, and the incredibly large gluing surface that the crenelated ends provide. The joint has incredible resistance to racking (bending of the joint), and even without glue the two boards can not be separated, except in the direction the pins were first inserted. This makes the joint ideal for high-stress uses such as attaching drawer fronts to the drawer cases: Every time the drawer is pulled open or slammed shut, the drawer front is undergoing extreme stresses. The dovetail joint ensures the drawer front can never separate from the case under normal usage.
The terminology of dovetails can be little confusing, and there are some variations. I will try to walk you through the basics before detailing some of the more innovative variations that are available.
The easiest way to remember which is which is to remember that it is the pins that do the sliding. The pin slides into the triangular space between the tails on the other board. A board will usually have multiple pins across its length that fit into matching spaces between tails on the other board. The pin or tail at the very outside edge of the board is either a "half-pin" or "half-tail" since it will only have a slope on one side.
The distance between the pins does not need to be uniform, however for visual appeal some form of pattern is highly desirable.
The angle of the pin and tail sidess must match if the pieces are to fit together, however the angle used can vary greatly. For most standard applications, an angle from 8º to 10º is ideal. If the angle is too steep, the ends of the tails are liable to break off during assembly or during the rigors of use. If the angle is too shallow, the ends of the tails do not exert enough pressure to hold in the pins, and the joint can be considered a box joint, which does not have the same mechanical benefits of dovetails.
This is the most basic form of dovetail. The end-grain of both pins and tails are visible from the outside surfaces of the joint since the pins go completely through the tails spaces, and the tails reach through the pins.
Decorative dovetails add even more visual appeal to the beautiful dovetail by varying the spacing, length, or shape of the pins and tails. These can be hand-cut, or they can be made using one of many router jigs. Some of the jigs available can even make exotic shaped pins.
Instead of using a standard half-pin and half-tail at the edge of the board, the half-pin and half-tail can be mitered to give the finished joint a clean mitered look when viewed from the edge, instead of the butt joint appearance it normally has.
Half-blind dovetails can only be seen from the side. This is because the pins are not cut completely through.
Traditionally, half-blind dovetails were cut by hand, though most are now cut using a router. Because of this, half-blind dovetails bear the stigma of being a machined joint, and even if the craftsman put in hours of work to hand-cut the joint, viewers would be hard-pressed to tell the difference. The evidence is undeniable prior to assembly though: routed half-blind dovetails have rounded slots between the pins and one side of the tails are rounded. Hand-cut half-blind dovetails have squared angles everywhere.
Full blind dovetails give all the mechanical strength of dovetails, without their outward appearance. The joint effectively appears as a mitered corner. This is achieved by not cutting the full depth on the pins or tails.
A slight variation on the blind dovetail is to create a lap on the tail board. The pin board is made as a half-blind pin board would normally be made, but the tail board is created with a lap that extends to cover the full end of the pin board, instead of being mitered as a full-blind dovetail would.
This joint is used to attach the end of one board to the middle of another board. Its advantages are that it allows for the removal of the sliding board, mechanically locks in the sliding board, and allows for natural wood movement.
The end of the sliding board receives a tongue with angled sides, and a matching slot is cut into face of the other board.
A variation of the standard sliding dovetail is the tapered sliding dovetail. The groove is slightly tapered, being wider where the tongue is inserted, and narrower at the far side. The dovetail tongue also receives a matching taper, leaving the tongue slightly thinner on the side that is inserted first into the slot. If this taper is accurately cut and matched on both pieces, the sliding dovetail produces a joint that has mechanical locking, and is held together tightly by the compression of the taper.
This joint is notoriously difficult to calculate how far the sliding board will travel before being stopped by the joint's taper. It is therefore recommended that you cut the sliding board slightly wider than required, then cut the dovetail tongue and the groove, test fit the pieces, then cut the sliding board to the size measured on the assembled project.
This is identical to a sliding dovetail joint however the slope is only on one side of the tongue and slot. In order to provide mechanical strength, the side of the board that receives the slope must be the same side of the board that will receive the force. For example, if a sliding half-dovetail is used on a horizontal shelf, books and other items are placed on the top of the shelf, and therefore the top of the tongue and top of the slot should receive the slope. Which side of vertical boards should receive the slope is slightly more difficult to determine, however a basic rule of thumb is that the slope should be on the side that is towards the center of the project if it is to provide maximum support.
1 Only if properly constructed. If both pieces have no grain like MDF or plywood, this is not an issue. For wood with grain, the matching dovetails must be made on matching sides with the same direction of grain. For example, end grain tongues matching to end grain grooves, or long grain tongues matching to long grain grooves. If you mix the grain directions (end grain tongues to long grain grooves), the expansion of one piece will be perpendicular to the expansion of the other piece, and the joint is liable to fail (separate or crack). Keeping the grain directions matched will prevent this.