The state of Doriathrin, as a language or dialect, relating to Sindarin proper is not fully known. In the annals the names Dolmed, Menegroth, Mablung, Nauglamîr, Dagnir Glaurunga, Thingol, Thuringwethil and Dior refer to names of the speech of Doriath. On the contrary, Daeron is proper Sindarin form of the Doriathrin name Dairon.
Doriathrin seems not to have been derived from Old Sindarin. For example OS *botto (from PQ *mbottô) brings proper Sindarin both; but the same word in Doriathrin is moth so it can't be derived from *botto but from an older form where the m of *mbottô was retained.
Another difference from Old Sindarin and Sindarin was that in general, Doriathrin had preferred short monosyllabic words where their Sindarin cognates had two syllables. Doriathrin tended to drop the second vowel. For example:
Other versions of the legendarium
Doriathrin is known only as a vocabulary from The Etymologies. Since the backstory of the Etymologies was different from the published Silmarillion, Doriathrin appears different from the Old Noldorin (read Old Sindarin); according to the later backstory, Sindarin and Doriathrin were brought into far closer contact than before because Old Sindarin (or a similar language) could have been the common ancestor of all the languages of Beleriand. Some[who?] would argue that the Doriathrin vocabulary became obsolete in the continuity of the later legendarium, and the words that "survived" were merged to proper Sindarin.