This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
66. Quoins are the corner stones of a wall, and are often dressed differently from the other stones, in order to make them more prominent, as, for example, those shown in Fig. 25. Quoin stones should always be equal in size to the largest stone used in the wall; otherwise, the effect of strength and solidity that they are intended to produce will be lost. Sometimes the quoins in a rubble-stone wall are built of brick, as shown in Fig. 21.
67. The stones in the sides of a door or window opening are called jamb stones. The alternate ones should extend through the width of the wall, to insure a good bond. Fig. 31 represents cut-stone jambs in a rubble wall; a shows the jamb stones bonding into the wall transversely; b, those bonding longitudinally; c, the stone window sill; and d, the rubble wall.
68. Occasionally, when stone piers or pilasters are built on the outside of a building, the windows are recessed so that the sills and lintels will not have as great a projection. This is shown in Fig. 32, in which a shows the lintel; b, the sash; and c, one of the jamb stones.
Jambs and quoins are often finished with a draft, or angle line, especially when the softer stones are used. Fig. 33