The aim of worship where such a set of beings is concerned, is to get hold of the spirit or god connected with the act one has in view, and so to deal with him as to avert his disfavour, which the Roman always apprehended, and gain his concurrence. The house-gods are beings possessing a stated cult, but outside the house-cult the worshipper has to face the question at each emergency which god he ought to address. He might choose the wrong one, which would make his act of worship vain. If he names the god correctly he will have a hold on him; in a case of uncertainty, therefore, he names a number of gods, in the hope that one of them will be the right one; or he invokes them all. "Whether thou be god or goddess" he will further say, if he is in doubt on that point, "or by whatever name thou desirest to be called." Each god has his proper style and title, and it is vain to approach him without these; lists of the various gods and of their correct styles were therefore drawn up in very early times to serve as guides to the subject. The Latin word "indigito," to point out, from "digitus," a finger, is the term used of addressing a god; the lists of deities with their proper appellations were called "indigitamenta"; and the gods named in them "Dii indigetes." The act of worship is grave and formal; it has to be done with precision and in strict accordance with the rules; silence is commanded; the sacrificer repeats the prayer proper for the occasion after some one who knows it by rote; the worshippers veil their heads. In this the Roman ritual is markedly different from the Greek. Mommsen says the Greek prayed bareheaded, because his prayer was contemplation, looking at and to the gods; and the Roman with head covered, because his prayer was an exercise of thought; and in this he sees a characteristic indication of the difference between the two religions. A more modern interpretation of the Roman practice is that it arose from the fear that the worshipper might see the god whom he has just summoned by name, which would be dangerous. If any mistake is made in worship, the act is vain and has to be done over again.