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.
Masonry walls (except those of concrete blocks) are seldom or never constructed entirely of single blocks which extend clear through the wall. The wall is essentially a double wall which is frequently connected by headers. These break up the continuity of the longitudinal vertical joints. The continuity of the lateral vertical joints is broken up by placing the stones of an upper course over the joints in the course below. Since the headers are made of the same quality of stone (or brick) as the face masonry, while the backing is of comparatively inferior quality, it costs more to put in numerous headers, although strength is sacrificed by neglect to do so. For the best work, stretchers and headers should alternate. This would usually mean that about one-third of the face area would consist of headers. One-fourth or one-fifth is a more usual ratio. Cramps and dowels are merely devices to obtain a more efficient bonding. An inspector must guard against the use of blind headers, which are short blocks of stone (or brick), which have the same external appearance on the finished wall, but which furnish no bond. After an upper course has been laid, it is almost impossible to detect them.
Amount of Mortar. For the same reasons given when discussing the relation of size of stones to amount of dressing required, more mortar per cubic yard of masonry is needed for small stones than for large. The larger and rougher joints, of course, require more mortar per cubic yard of masonry. In the tabular form at top of page 95, are given figures which, for the above reasons, are necessarily approximate; the larger amounts of mortar represent the requirements for the smaller sizes of stone, and vice versa:
The stones should be very thoroughly wetted before laying in the wall, so that they will not absorb the water in the mortar and •ruin it before it can set. It is very important that the bed-joints should be thoroughly flushed with mortar. All vertical joints should likewise be tightly filled with mortar.