This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
From the modified butt joint it is only a step to the "mortise-and-tenon" joint, which is formed by cutting a hole called a "mortise" in one of the pieces of timber, to receive a projection called a "tenon" which is cut on the end of the other piece. This arrangement is shown in Fig. 41. The mortise is shown at A, and the tenon is shown at B. It will be noticed that there is a hole bored through the tenon at C, and that another hole is bored in the mortised piece at D. These holes are so placed that when the pieces are joined together, a wood pin may be driven through both holes, thus preventing the tenon from being withdrawn from the mortise. The pin should always be inserted in a mortise-and-tenon joint. Ordinarily this pin is of hard wood, even when the pieces to be joined together are themselves of soft wood, and it may be of any desired size. Round pins from 3/4 inch to 7/8 inch in diameter are ordinarily employed, although it may sometimes be found better to use a square pin.
The form of mortise-and-tenon joint described above may be used wherever the pieces are perpendicular to each other. When, however, the pieces are inclined to each other, a modification of the above joint known as the "bridge" or "straddle" joint is employed. This joint is shown in Figs. 42 and 43. It is similar to the square mortise-and-tenon joint, having a similar mortise and tenon, but these are cut in a slightly different way. In Fig. 42 the tenon A is cut in the end of the inclined piece and fits into the mortise B cut in the other piece. In Fig. 43 the mortise A is cut in the end of the inclined piece and the tenon B is cut in the other piece.
Fig. 41. Square Mortise-and-Tenon Joint.
Fig. 42. Form of Bridge or Straddle Joint.
Fig. 43. Another Form of Bridge or Straddle Joint.