This research project brings together historical and typological linguistics to address a central issue in the field: the classification of parts of speech across the world’s languages. Its primary research areas include the typology of parts of speech, Indo-European and Semitic historical linguistics, linguistic terminology, theoretical morphology and the history of linguistics. The project pursues two main objectives: on the one hand, to develop a coherent theory of part of speech that is applicable across languages; on the other, to shed light on a largely understudied typological shift—namely, the lexicalization of the adjective class within the two language families under investigation.