The name is a compound of the Latin words for pig, sheep and bull, and it describes the offering exactly: one of each, male, driven together in procession. What the rite does is enclose. The three animals are led round the perimeter of whatever is to be purified, and only after the circuit are they killed and offered. Everything inside the line the procession has walked is covered; everything outside it is not. That is why the same form served a smallholding, a legion drawn up on a parade ground, a city and, in the census, the entire Roman people. The rite scales by walking further.
Cato preserves the farmer's version in his manual on agriculture, and it is the fullest early description of any Roman rite. The animals are led round the land; wine is offered; and a prayer is addressed to Mars in the imperative language Roman prayer used with the gods, asking him to keep away and drive off disease seen and unseen, dearth and desolation, storms and bad weather, and to keep the crops, the vines, the plantations, the shepherds and the flocks in good health. The prayer names alternatives and covers contingencies with the thoroughness of a legal instrument, and Cato adds instructions for what to do if the omens are unfavourable.
The word for the rite and the word for purification are the same root, and the public version gave the Latin language a unit of time: because the census closed with a lustration and the census came round every five years, a lustrum came to mean the five year interval itself.