The Roman caeremonium is composed of a number of stages, each stage composed of one or more rituals. The arrangement of these stages, and the order in which they may appear, can vary according to the kind of caeremonium. Here I follow a model posed by John North of University College London for the structure of Roman ritual, on which I have expanded some of the details.
I. – PRAEFATIO
The preliminary stage of a formal Roman ritual begins with a praeco (herald) calling for people's attention by announcing "Hoc age!" The main celebrant then appoints his assistants, with the most important assistant being the minister sacrificii. An important feature in a Roman ceremony is the dialogue that takes place between the celebrants. The main celebrant, called the praesus or the praesul, is responsible for ordering or authorizing the other celebrants to do their parts. No action is taken during a ceremony without a person asking permission and the praesul authorizing him to proceed. An example from a private ritual is where Cato the Elder wrote:
"The proper way to purify the grain fields is in this manner. Order a piglet, a lamb, and a calf to (suovitaurilia) be led around, using these words: 'With the favor of the Gods, everything may turn out well, so I bid you, Manius, to take care to purify my farm, my field, my land with this suovetaurilia, may you lead or carry as many of the sacrificial victims as you wish aroundwhatever part of my estate, my field, and my land as you think best (De Agricultura 141)."
Another example is give by Cicero in the case of taking auspices. Here the praesul asks, "Quintus Fabius, I wish you to be (an assistant) to me at the auspices." To this, Fabius answers, "Audivi." "(Thus) I have heard, (De Divinatione 2.71)."
A ritual purification is then made by the praesul with the assistance of a camillus. This was a child, which for Romans could be anyone up to the age of thirty, but was usually a prepubescent boy or girl whose parents were still living and still married to one another. By law, the camillus had to hold a bowl in his or her left hand and with the right foot forward for good luck the camillus would pour pure water from a vessel over the hands of the praesul, who in turn would sprinkle the water three times over his forehead.
An initial sacrifice of incense, wine, and possible a cake of grain, might be offered at this time to call upon certain deities to act as witnesses to the initial rituals.
II. – POMPA
The pompa is a procession. It is a ritual all to itself. Pompae are arranged differently for military triumphs, funeral processions, for when games were being held in honor of the Gods, and for other kinds of occasions. That is, the arrangement of a pompa, how the various components of the procession were placed, characterized what type of ritual was being celebrated. In general, though, you may think of it as the procession in which offerings are carried or led to the altar. I shall cover the pompa in more detail in a later post.
More at Structure of a Formal Roman Ritual | Religio et Pietashttp://www.patheos.com/blogs/religioromana/2012/01/structure-of-a-formal-roman-ritual/