File:Grooved expansion joints at the Ishtar Gate.png

English: (The foundations of the main building are deep but the gateway wings are not deep.) "It is conceivable that those parts of the wall where the foundations are specially deep do not sink so much in the course of time as those of shallower foundations, and settlement is unavoidable even with these, standing as they do upon earth and mud. Thus where the foundations are dissimilar there must be cleavages in the walls, which would seriously endanger the stability of the building. The Babylonians foresaw this and guarded against it. They devised the expansion joint, which we also make use of under similar circumstances. By this means walls that adjoin each other but which are on foundations of different depths are not built in one piece. A narrow vertical space is left from top to bottom of the wall, leaving the two parts standing independent of each other. In order to prevent any possibility of their leaning either backwards or forwards, in Babylon a vertical fillet was frequently built on to the less deeply rooted wall, which slid in a grove in the main wall (Fig. 22). The two blocks run in a guide, as an engineer would call it."