The Forest of Brethil was between the Rivers Teiglin and Sirion. It was technically a part of the Realm of King Thingol of Doriath, though it was not within the Girdle of Melian.

After hearing from Finrod Felagund of all that had befallen the lady Haleth and her people, Thingol agreed to give the Forest of Brethil to the People of Haleth under one condition: that the people who dwelt there should guard the Crossings of Teiglin against all enemies of the Eldar, and allow no Orcs to enter their woods.

They agreed and so it was that some of the Edain were allowed to intermingle their homes with those of the Eldar, for a time.
Encyclopedia entry originally written by Nienna-of-the-Valar