This paper analyses the diachronic development of English HAD BETTER (and HAD RATHER) structure(s). It is argued that the original construction out of which HAD BETTER/RATHER developed and which contained the verb BEON ‘be' could be substituted by a new construction employing HAD by virtue of the fact that these two verbs in a great number of contexts are devoid of any cognitive content leading to their mutual interchangeability (§1). Section 2 is devoted to the examination of the development of the construction in question. In section 3 it is shown how the mechanisms and principles of grammaticalisation set out by Lehmann (1982) (002), Hopper (1991) and Heine (2003) (2005) apply in this particular case of grammaticalisation. The bulk of language illustrations comes from the Dictionary of Old English, the Corpus of Middle English Prose and Verse and the Helsinki Corpus of English Texts. The references to the actual works follow the pattern of the first two electronic corpora.