The name of an English phonological rule. A root morpheme containing a long vowel or diphthong has this replaced by a short vowel when the relevant syllable falls at least three from the end in a derived form.

Some examples:

• iː → e E.g. serene ~ serenity
• eɪ → æ E.g. sane ~ sanity
• aʊ → ʌ E.g. profound ~ profundity
• aɪ → ɪ E.g. derive~ derivative
• əʊ → ɒ E.g. provoke ~ provocative

The phenomenon is fairly general, but there are exceptions. obesity = əˈbiːsəti instead of the expected *əˈbesəti is an example.