1. The semantics of modal verbs;Boyd;Journal of Linguistics,1969
2. The Semantics of the Modal Auxiliaries;Coates,1983
3. The Meanings of the Modals in Present-day American English;Ehrman,1966
4. Relevance and modality;Gardón;Revista Alicantina de Estudios Ingleses,1998
5. Can, may, must and should;Groefsema;Journal of Linguistics,1995