A declaração mais clara disso é dada no Apêndice do Senhor dos Anéis F:
In the years of their power the Númenoreans had maintained many forts and havens upon the western coasts of Middle-earth for the help of their ships; and one of the chief of these was at Pelargir near the Mouths of Anduin. There Adûnaic was spoken, and mingled with many words of the languages of lesser men it became a Common Speech that spread thence along the coasts among all that had dealings with Westernesse.
After the Downfall of Númenor ..... They used therefore the Common Speech in their dealing with other folk and in the government of their wide realms; but they enlarged the language and enriched it with many words drawn from Elven-tongues.
In the days of the Númenorean kings this ennobled Westron speech spread far and wide, even among their enemies; and it became used more and more by the Dúnedain themselves, so that at the time of the War of the Ring the Elven-tongue was known to only a small part of the peoples of Gondor...
Em relação a origens ainda mais antigas, o Apêndice F novamente fornece a prova:
It was in origin the language of those whom the Eldar called the Atani or Edain, 'Fathers of Men', being especially the people of the Three Houses of the Elf-friends who came west into Beleriand in the First Age...
Assim, a língua que se tornou o Westron foi originalmente falada pelos Homens das Casas de Beor, Hador e Haleth, foi levada para Númenor onde se tornou Adûnaic, e eventualmente trazida de volta à Terra-média durante os anos do poder Númenoreano. evoluiu ainda para o Westron da época do Senhor dos Anéis.