Butt joints unite the end grain of a piece of wood to the long grain of another piece to form a corner. They're easy to assemble, but they require strengthening with gussets, nails or dowels.
When using nails to secure a butt joint, a vise can be used to make the job easier. Start by driving two nails at one end of the width of a block. Drive the nails at an angle so that if one is pulled, the other nail will pull in tighter. Don't drive the nails all the way through: give them about three taps into the wood. Use a vise to hold a second block in a vertical position. Place the nailed block perpendicular on top of the vertical block, and finish driving the nails through the two blocks.
A gusset is a triangular piece of wood that fits over the top of a butt joint to strengthen it. Gussets are made by cutting a piece of square plywood into two right triangles. Glue the gusset into place over the joint with its longest side facing in. Drive nails into each corner of the gusset to secure it.
Two dowels may be used to form a strong butt joint. To make this type of joint, hold two blocks together, and mark the points where you want the two dowels to go. Drill holes into the two marks on the width of one block and on the end of the length of the other block. The holes must be the same size as the dowels.