As it was necessary to protect the homestead and its land by a sacred boundary, so the city had to be clearly marked off from all that was outside of it. Its walls were sacred, or, strictly speaking, a certain imaginary line outside of them called the _pomoerium_ was sacred. This is well shown in the traditional method of founding a city even in historical times, _e.g._ a _colonia_, as described by Varro, Servius, and Plutarch.[450] A white ox and a white cow were harnessed to a plough, of which the share must be made of bronze--a rule which shows at once the antiquity and the religious character of the rite, for iron, as we saw, was taboo in most religious ceremonies. A rectangular furrow was drawn where the walls of the city were to be; the earth was turned inwards to mark the future line of the wall, and the furrow represented the future _pomoerium_. When the plough came to the place where there was to be a gate, it was lifted over it, and the ploughing resumed beyond it. This probably meant, as Plutarch expressed it, that the walls (or rather the _pomoerium_), were sacred while the gates were profane; had the gates been holy, scruple would necessarily have been felt about the passage in and out of them of things profane. Thus the _pomoerium_ was a boundary line between the sacred and the profane, like that of the farm; but in historical times it acquired a more definite religious meaning, for within it there could only dwell those deities who belonged to the city and its inhabitants, _i.e._ the _di indigetes_, and who were recognised as its divine inhabitants.[451] And only within its limits could the _auspicia_ of the city be taken.