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The Roman Senate kept tight control over the "Sibylline Books", and entrusted them to the care of two patricians. In 367 BC, the number of custodians was increased to ten, five patricians and five plebeians, who were called the "decemviri sacris faciundis". Subsequently, probably in the time of Sulla, their number was increased to fifteen, the "quindecimviri sacris faciundis". They were usually ex-consuls or ex-praetors. They held office for life, and were exempt from all other public duties. They had the responsibility of keeping the books in safety and secrecy. These officials, at the command of the Senate, consulted the "Sibylline Books" in order to discover not exact predictions of definite future events in the form of prophecy, but the religious observances necessary to avert extraordinary calamities and to expiate ominous prodigies (comets and earthquakes, showers of stones, plague, and the like). It was only the rites of expiation prescribed by the "Sibylline Books", according to the interpretation of the oracle that were communicated to the public, and not the oracles themselves, which left ample opportunity for abuses.