One of the tidiest and strongest ways of securing the horizontal rails to a fence post is to mortise the rails. A mortise is a cut opening into which a piece of wood, commonly referred to as a "tongue," is inserted. A mortise if far stronger than a surface mounting and more attractive than any type of bracket. The method for cutting a mortise into a fence post is common to a mortise of any size for any type of fence rail board.
Lay out the desired location of the mortise opening on the side of the fence post. Use a hand square to connect the lines to form a square outline for the mortise hole.
Cut the perimeter line of the mortise using a wood chisel. Place the flat side of the chisel tip on the line and tap the chisel with a hammer or wood mallet. Repeat the action until the full perimeter of the mortise has been scored.
Chip out the interior area of the mortise outline, cutting toward the inside and pivoting the chisel to remove the wood chips. Chip out a minimum of 1-inch and a maximum 2-inch depth for proper fence rail support.
Repeat Steps 1 through 3 to complete the mortising of all the fence posts, as needed.