[...] a word may have the character of a secondary derivative and yet consist of only one morpheme, accompanied by a zero-feature ('sheep', as a plural; 'cut', as a past) by a substitution-feature ('men', 'sang'), by suppletion ('went', 'worse'), or by a minus-feature (French 'vert', masculine). We class these words as secondary derivatives and recognize their peculiarity by calling them 'secondary morpheme-words'.
- Bloomfield (1935), Pag. 218