Dowel joint

A dowel joint pins two pieces together with short round wooden pegs glued into matching holes drilled in both. A doweling jig holds the drill square to the face and puts the holes in the same place on each piece, so the parts line up when the dowels go in.

Why a dowel helps at end grain

The USDA Wood Handbook says a plain end-grain butt joint can't be glued strong enough for ordinary service, and that a joint where end grain meets edge grain needs interlocking parts so that edge grain is glued to edge grain. It names dowels, along with mortise and tenons and rabbets, as examples of strong connections of that kind.

What size dowel

A dowel is traditionally about one third the thickness of the stock it joins. For 3/4 in stock that is 1/4 in. The dowel size chart works the rule for common thicknesses and lists the diameters and lengths three makers sell. Hole depth and the glue allowance below the dowel are left to your jig's manual: the references we read give different figures.