Among all the gods worshipped by the Greeks, the twelve deities who dwelt on Mt Olympus, the highest mountain in Greece, formed a special category of their own. The gods of Olympus were usually taken to be Zeus, Hera, Athena, Poseidon, Apollo, Artemis, Demeter, Hermes, Aphrodite, Ares, Hephaestus and Hestia. In certain local variations, positions among the 'twelve' were occupied by Pluto (Hades), Dionysus, Heracles or other local cult heroes. The twelve gods - six male and six female - were divided in accordance with their properties and activities into six couples united by bounds of friendship or kindship; Zeus/Hera, Poseidon/Demeter, Apollo/Artemis, Hermes/Athena, Ares/Aphrodite, Hephaestus/Hestia.

The gods of Olympus lived mostly in peace, in a home which was both idyllic and imposingly majestic. In choosing to dwell on Olympus, they were on the borderline between earth and sky, and could thus supervise everything that the creatures of the world were doing. Their main object of interest was man, whom they sometimes showered with gifts and sometimes persecuted. Their attitude to one another was somewhat similar: terrible crises might break out in their relations, but there were many occassions on which as loving companions they caroused together at sumptuous banquets, eating ambrosia and drinking nectar - the sole sustenance of the gods. The leader of the gods was Zeus the omnipontent, followed by the others in a rudimentary hierarchy in which each deity has a sphere of influence of his or her own.

Whatever the powers of the twelve gods might be, none of them had the right to break the sacred oath they had given by the waters of the Styx. Styx, the daughter of Oceanus and Tithys, personified the sacred river of the Underworld and had been specially honoured by Zeus for the part she had played in the Battle of the Giants. The gods thus swore by the waters of the Styx, and if they happened to break a promise they had made, their punishment was to spend a year in a kind of coma, after which they were banned from the meetings of the other deities for a further nine years.

If the gods swore by the Styx, the gravest oath for mankind was 'by the twelve gods'. The penalty for breaking this oath was particularly severe, since apart from anything else the gods of Olympus were a kind of supreme court for human offences - one which had to be treated with absolute respect and unflagging care to ensure that the proper honours were paid to it. The ancient Greeks worshipped each god separately, and also set up altars to all the Twelve Gods in the very centres of their cities. The Altar of the Twelve Gods in Athens, in particular, was the exact centre of the city, and all distances were measured from it.