The Thracian language in linguistic textbooks is usually treated either as its own branch of Indo-European, or is grouped with Dacian, together forming a Daco-Thracian branch of IE. Older textbooks often grouped it also with Illyrian or Phrygian. The belief that Thracian was close to Phrygian is no longer popular and has mostly been discarded."We will dismiss, at least temporarily, the idea of a Thraco-Phrygian unity. Thraco-Dacian (or Thracian and Daco-Mysian) seems to belong to the eastern (satem) group of Indo-European languages and its (their) phonetic system is far less conservative than that of Phrygian (see Brixhe and Panayotou 1994, §§ 3ff.)"</ref> The Thraco-Illyrian grouping has also been called into question. Daco-Thracian or Thraco-Dacian is the main hypothesis.