The Sumerian language is generally regarded as a language isolate in linguistics because it belongs to no known language family; Akkadian, by contrast, belongs to the Semitic branch of the Afroasiatic languages. There have been many failed attempts to connect Sumerian to other language groups. It is an agglutinative language; in other words, morphemes ("units of meaning") are added together to create words, unlike analytic languages where morphemes are purely added together to create sentences. Some authors have proposed that there may be evidence of a sub-stratum or add-stratum language for geographic features and various crafts and agricultural activities, called variously Proto-Euphratean or Proto Tigrean, but this is disputed by others.