Cyril Bailey, The Religion of Ancient Rome
“ Take first of all the performance of the regular sacrifices or acts of worship ordained by the state-calendar or the celebration of the household sacra. The pietas of man consists in their due fulfilment, but he may through negligence omit them or make a mistake in the ritual to be employed. In that case the gods, as it were, have the upper hand in the contract and are not obliged to fulfil [20] their share, but the man can set himself right again by the offering of a piaculum, which may take the form either of an additional sacrifice or a repetition of the original rite. ”
