Full Idea
Hare says words are secondarily evaluative (e.g. 'soft-hearted') if prescriptive meaning varies but description is constant; primarily evaluative words ('good', 'right', 'ought') are the opposite, with the descriptive content varying.
Gist of Idea
In primary evaluative words like 'ought' prescription is constant but description can vary
Source
report of Richard M. Hare (The Language of Morals [1952]) by Brad W. Hooker - Prescriptivism p.640
Book Reference
'Cambridge Dictionary of Philosophy', ed/tr. Audi,Robert [CUP 1995], p.640
A Reaction
I would have thought that the prescriptive meaning of the evaluative word could at least vary in strength. You really, really ought to do that.