The teams of masons at work on the dome had certain basic measuring
devices at their disposal. Most of these had not changed significantly for a
thousand years. For checking the perpendicularity of walls, for example,
they used plumb-lines: a string on which a weight, usually a ball of lead,
was suspended. The string would be braided like a fishing line in order to
prevent the weight from rotating in the breeze. And to ensure the stones
were laid in perfectly horizontal courses or layers, a mason's level was
employed. This instrument was shaped like the letter A: a plumb-line hung
from its apex, while the horizontal cross-piece was inscribed with a
graduated scale. The plumb-line would come to rest in the centre of the
cross-piece when the stone or brick was on a level plane.