‹‹‹ Vitr. 2.8.5 | Table of Contents | Vitr. 2.8.7 ›››
6The first is so called, because in it all the courses are of an equal height; the latter received its name from the unequal heights of the courses. Both these methods make sound work: first, because the stones are hard and solid, and therefore unable to absorb the moisture of the mortar, which is thus preserved to the longest period; secondly, because the beds being smooth and level, the mortar does not escape; and the wall moreover, bonded throughout its whole thickness, becomes eternal.
6A wall is called isodomum when all the courses are of equal height; pseudisodomum, when the rows of courses do not match but run unequally. Both kinds are strong: first, because the rubble itself is of close texture and solid, unable to suck the moisture out of the mortar, but keeping it in its moist condition for a very long period; secondly, because the beds of the stones, being laid smooth and level to begin with, keep the mortar from falling, and, as they are bonded throughout the entire thickness of the wall, they hold together for a very long period.