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.
107. When there are window or door openings in a foundation wall, the stones under the opening should be laid as shown at a, b, c, Fig. 40. This is done to spread the weight of the wall under the door or window. If there should be a great amount of weight resting on the foundation walls, the window sills should not be built in the wall, but should be slip sills, as they are called, or sills that are just the width of the opening, as shown at d, Fig. 40. There will then be no danger of breakage by reason of uneven settlement of the wall; otherwise, if the wall settles more on one end of a built-in sill than on the other, the sill will probably crack.