The Beleriand river Taeglin was a tributary of Sirion.
It rose in the Ered Wethrin and flowed southeast through Brethil to join Sirion on the borders of Doriath.
An important for on the river was the Crossings of Teiglin, near the western border of Brethil. A road ran through it from Nargothrond to Minas Tirith.
Minor tributaries were Celebros and Malduin.
Other versions of the legendarium
In earlier writings, the river was called Teiglin. As revealed in the The History of Middle-earth series, the rivername should actually have been spelt Taeglin ("Marking River"). This was a relatively late change which was not adopted into the published The Silmarillion, which uses the old name.