Before the Second Age, Belegaer stretched from the Gap of Ilmen in the far north, where a bridge made of ice known as the Helcaraxë connected Middle-earth and Aman, to the far south, where it also connected with Ilmen and froze, although no strait was formed there. Belegaer was narrower in the north than in the south, with its widest part near the girdal of Arda.
The full extent of Belegaer after the Akallabêth is never made clear, but it reaches far enough to the north to be ice-covered, and far to the south. Belegaer corresponds with (and is intended to be) the Atlantic Ocean.
The name is Sindarin, and has the elements Beleg or Might and Aer or Eär, an element meaning Sea which is also present in the name Eärendil (sea-lover). The Quenya name of Belegaer, never used in published writing, is Alatairë.
Before the end of the Second Age, the continent of Aman, home of the Valar, formed the western edge of Belegaer. Before the ruin of Beleriand at the end of the First Age, the sea was narrow and ice-filled in the north, forming the strait of Helcaraxë, the Grinding Ice. It was thus possible to cross from Aman to Middle-earth on foot, though with difficulty, as did Fingolfin and his people of the Ñoldor when fleeing Valinor.
After the War of Wrath Belegaer was widened by the drowning of a large part of Middle-earth, and possibly parts of Aman. The bridge of ice in the north was removed, removing land access to the western continent. During the Akallebêth in the Second Age, the seas were bent and the world was made round. Aman was removed from the world, Belegaer washed "new lands", and only the chosen could find the "Straight Road" to Valinor. The new western end of Belegaer is never described in the narrative, although there are indications that Númenórean refugees reached them in search for Valinor.
Belegaer has and had several islands and island chains:
- Tol Eressëa, the Lonely Isle close to the shore of Aman
- the Twilit Isles or Enchanted Isles seaward of Tol Eressëa
- Númenor (Second Age only)
- the Western Isles (after the First Age)
- Balar, in the Bay of Balar (First Age only)
- Tolfalas, in the Bay of Belfalas
Other features of Belegaer: