Tyr is widely thought to be, in origin, a very ancient sky- and war-god, his name descends from the same Indo-European root as Greek Zeus and Latin Jupiter (from Dyeus), and the Old Norse word tyr could be used as a common noun meaning simply a god. This suggests that Tyr was once far more central, perhaps a supreme sky-father, before Odin and Thor overshadowed him in the mythology that survives. His antiquity shows in the calendar: the Germanic peoples used him to render the Roman god of war, so that dies Martis, Mars's day, became Tiw's day, English Tuesday. He remained the god invoked for victory, law and the sworn oath, the just and courageous power behind both the battlefield and the assembly.
Tyr's antiquity is documented outside the Norse sources too: the Roman historian Tacitus, writing his Germania around 98 CE, describes the Germanic peoples honouring a god he calls Mars above their other war-gods, in keeping with the Roman habit of naming foreign deities after their nearest Roman equivalent, and scholars generally read this Mars as an early form of Tyr, centuries before the Icelandic sagas were composed. The rune named for him, Tiwaz, shaped like an upward arrow or spear-point, was carved for victory in battle; the Old Norse poem Sigrdrifumal instructs a warrior to cut Tyr-runes twice into a sword hilt and invoke the god's name to win in combat, a piece of documented runic magic tied directly to his name.