Affix Ordering Generalization

Affix Ordering Generalization is a generalization over class I and class II affixes which entails that class II affixes can attach to words derived with class I affixes, but not vice versa.

Example
According to Siegel (1974) class I affixes in English such as -ion, -ity, -al and -ive trigger and undergo phonological processes, while class II affixes such as -ness, -less, -ful and -ly do not (e.g. op[ei]que: op[æ]city: op[ei]queness (Trisyllabic Shortening),párent: paréntal: párentless (Stress shift)). Next to these differences it appears that class I affixes cannot appear outside class II affixes (*hopefulity).

Comment
Some well-known exceptions to the Affix Ordering Generalization are discussed in Aronoff (1976).

Link
Utrecht Lexicon of Linguistics