Halliday & Matthiessen (1999: 299):
… one property of language which is universal is the stratal organisation of the content plane into semantics and lexicogrammar, with the lexicogrammatical stratum forming a continuum: at one pole are the most "grammatical" features, closed systems of just two or three terms, mutually defining along a single dimension and with very general meanings and contexts of use; at the other pole are the most lexical features, open sets of an indefinite number of items, taxonomically arranged along various dimensions and with highly specific meanings and contexts of use.