Halliday & Matthiessen (2014: 66):
But if grammar and lexis are interpreted as the endpoints of a continuum, what lies in between them, around the middle? It is here that we locate those items that, on the paradigmatic axis, enter into series which could be regarded from both angles of vision: either, in a grammatical perspective, as rather large and fuzzy closed systems or, in a lexical perspective, as somewhat determinate and limited open sets. This would include, in English, things like prepositions, temporal and other specialised adverbs, and conjunctions of various kinds.