Who were the twelve Olympians, and why was the list never quite the same?

The twelve Olympians were the great gods of the Greeks: Zeus, Poseidon, Hera, Demeter, Aphrodite, Athena, Artemis, Apollo, Ares, Hephaestus, Hermes, and either Hestia or Dionysus. They were called Olympians because, according to tradition, they lived on Mount Olympus.

Notice the "either". The number was fixed at twelve, but there was never one official roster. Hestia and Dionysus competed for the last seat, and shrines to "the twelve gods" in different cities listed different members. One list from Olympia even included the Titans Cronus and Rhea, who were not Olympians at all.

How did these twelve come to rule? The Olympians were a younger generation of gods. The first of them, Zeus, Poseidon, Hera, Demeter and Hestia, were children of the Titans Cronus and Rhea. After a ten-year war, Zeus led his siblings to victory over the Titans and the Olympians took charge of the world.

Not every major god got a seat. Hades was Zeus's brother and a powerful deity, but his kingdom was underground, far from Olympus, so he was usually left off. Gods of the underworld were even honoured differently: their offerings went into a pit or a sunken chamber instead of onto an altar.

The Athenians gave the twelve a famous altar in their marketplace, and it became the point from which distances from Athens were measured.