Words as units in the lexicon are the subject matter of lexicology. While words are generally accepted as being (with clitics) the smallest units of syntax, it is clear that, in most (if not all) languages, words can be related to other words by rules. The rules understood by the speaker reflect specific patterns (or regularities) in the way words are formed from smaller units and how those smaller units interact in speech. In this way, morphology is the branch of linguistics that studies patterns of word-formation within and across languages, and attempts to formulate rules that model the knowledge of the speakers of those languages, in the context of historical linguistics, how the means of expression change over time. See grammaticalisation.