When two masonry walls intersect in modern buildings, the most common visible occurrence is at an external corner. Concrete blocks are square in section and twice as long as they are wide. This permits good bonding at those exterior corners, as shown in these illustrations, with the courses alternately passing through the corner.

One wall course.

The bonding wall course with the first course.

The first two courses with a second course on one wall only.

The first two courses.

The first two courses with the third course on one wall only.

The first three courses.

The first three courses with the fourth course on one wall only.

The first four courses.

This kind of bonding at the corner is standard in concrete block or brick construction, and the blocks are sized to facilitate this kind of bond. Where interior walls abut exterior, continuing walls, there may or may not be a real connection between the walls. Often the cross-walls are left more or less free-standing. If they are bonded, though, half-length blocks are used to permit the intersections to avoid interrupting normal blocks patterns, and only alternate courses will actually bond. The resulting bond is shown in these drawings. Note that a half-length block is required only in the exterior, continuing course where the walls bond.

One wall course.

The continuing wall course with the first bonding course.

The first two courses with a second course of the continuing wall only.

The first two courses.

The first two courses with the third course of the continuing wall only.

The first three courses.

The first three courses with the fourth course of the continuing wall only.

The first four courses.

When two walls intersect in a monumental Greek building constructed with ashlar masonry, they rarely do so at a closed corner such as the one illustrated first. That is, the major walls of monumental buildings tend to end in antae, and cross-walls meet those major walls somewhere in the middle, not at a corner. (An in antis plan, by definition, yields a facade wall behind the columns and closer to the interior than the antae. A prostyle porch leaves antae as well, with the cross-wall barring the way to the cella just behind the antae. A plan of the prostyle Parthenon is available at Wikipedia. Though it is not detailed enough to show antae, it does show the side walls continuing beyond the cross-walls of the cella and opisthodomos.) Similarly, the Propylaea (see http://propylaea.org/propgeninfo.html for a plan), though more complex in terms of the number of wall joints, has no exterior corner formed by two walls other than the closed corner at the NW corner of the NW wing. Thus, in such monumental buildings, the normal procedure is for one wall - the one we would call the cross-wall in most instances - to seem to abut the other, continuing wall. The joint is more complicated, however, since the cross wall always bonds with the longer, continuing wall so that each supports the other more fully.

Visually, one might think that these cross-walls do not bond with the continuing walls, because monumental Greek buildings do not show, in the exterior wall-block pattern of the continuing wall, any evidence of the existence or position of the cross-wall. The cross-wall terminates within the depth of the continuing wall; it does not pass through the full depth of the wall as a modern concrete block or brick does. Thus, the cross-wall penetrates the continuing wall to bond with it, but only to some depth less than the thickness of that wall. How deeply the cross-wall goes into the continuing on any given joint of any given building wall cannot be determined with certainty unless the structure is dismantled.

This bonding with the cross-wall only partially penetrating the continuing wall requires that the continuing wall include blocks that have been cut away in the proper locations for receiving cross-wall blocks. Those blocks in the continuing wall could be cut so as to permit the cross-wall block to penetrate more or less in the center; they could also be cut at one end only so that the face of the cross-wall aligns with one edge of the block in the continuing wall. While both approaches may be somewhat complicated by ancient practices of wall inclination, the latter - letting the cross wall blocks bond at the edges of continuing-wall blocks - reduces the amount of labor required by a significant margin. It also permits the visible interior corner to "read" as if the block pattern were continuous to the corner rather than changing in the last block.

It must be stressed that any solution to the problem of bonding walls also involves the chosen dimensions of the wall blocks. That is, if wall patterns are to be seen as uninterrupted by the corner, keeping that appearance on the other side of the cross-wall will depend on the dimensions of the blocks. Moreover, the consistency of block dimensions obviously impacts the whole plan. In general, though, courses in a Classical-era wall include only blocks of the same size (that is, intended to be the same, though possibly varying by a cm or two) unless there are interruptions such as doors or other openings requiring adjustment.

The following illustrations should help with visualizing the issues. The blocks drawn are simply shaped, with each block square in section and twice as long as it is wide. As is apparent in the drawings, though, the shapes/proportions of the blocks will have a strong impact on the sense of the coursing, as will the extent to which a cross-wall block penetrates the continuing wall. That is, if the blocks were all thicker than half their length, it might be possible to penetrate the continuing wall deeply enough to make the coursing of the cross-wall match the coursing of the continuing wall - all joints lying in the middle of the block above and the block below. The chosen dimensions do, however, permit the coursing in the continuing wall to "read" as continuous on both sides of the cross-wall.

The drawings show two versions of a continuing wall and partially-penetrating cross-wall, with the cross-wall meeting the continuing wall at the end of a continuing-wall block (right) or in the middle (left).

The first continuing-wall course.

The first continuing-wall course with the first cross-wall course.

The first two continuing-wall courses with the first cross-wall course.

The first two continuing-wall courses with the first two cross-wall courses.

The first three continuing-wall courses with the first two cross-wall courses.

The first three continuing-wall courses with the first three cross-wall courses.

This series of drawings shows a different wall, with blocks square in plan, as wide as they are deep. This is, I believe, the only scheme that will permit truly uninterrupted coursing on both sides of a cross-wall. In other words, unless the blocks are square in plan, the coursing must be different on the two sides of a cross-wall. Since blocks are not usually square, as a result, coursing will not reach this quasi-ideal state, with all joints aligning with block centers. (The situation is not so stark with the cross-wall. Any dimensional choices permitting cross-wall blocks to penetrate the continuing wall by half their length - whether by altering the block dimensions or by altering the depth of the penetration - will permit the cross-wall jointing to be regular and permit joints to fall in line with block centers.

The first continuing-wall course.

The first continuing-wall course with the first cross-wall course.

The first two continuing-wall courses with the first cross-wall course.

The first two continuing-wall courses with the first two cross-wall courses.

The first three continuing-wall courses with the first two cross-wall courses.

The point of all this time spent discussing bonds between wall courses is simply to make it clear that even so simple a matter as this is not simple. An architect or mason must know how the walls will bond and what the block dimensions will be in order to plan a structure. If you have not read the information about the Arsenal of Philon, it is a good illustration of just how fully the architect did understand the implications of his wall bonds and block sizes.

It is also important to see how little information we may have about the blocks, a critical problem since we have so little to go on. Given the fact - discussed in "contract Terms" - Four-stage shaping of blocks - that we can only be sure of block lengths from a finished building, our information base is slim indeed.