This article argues that it is frequency and usage effects, arising in particular contexts and constructions, that lead to the emergence of new semantic and syntactic properties in expressions that become discourse markers (DMs). These changes, it is argued, are comparable to those that take place in the development of other grammaticalizing linguistic items. DMs are taken to be sentence adverbials that express discourse-relational predications. Data from the histories of two English DMs, ‘instead’ and ‘rather’, are examined. These contrastive DMs are often interchangeable in present-day English but have very different origins, ‘instead’ deriving from the phrase ‘in (the) stead of NP’, and ‘rather’ from the comparative VP-adverb rather, meaning ‘sooner’ or ‘faster’. In each of the two cases, there is seen to be increased type frequency (with context expansion) and reanalysis (wider scope). The diachronic evidence supports the idea that the reanalysis resulted from wider interpretation, not from prior change in word order. As DMs, both ‘instead’ and ‘rather’ become associated with high informational salience. Both undergo functional split, leading to new polysemies. Insofar as the newer, DM senses are closer to the grammatical end of the lexical-grammatical cline, the expressions can be said to have grammaticalized. There is no evidence that any qualitatively special changes are involved in the emergence of the DM uses, compared with the developments of the other polysemies of the expressions.
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