In some cases of morphemes having one environment but not another in common, both the similarity and the difference are relevant for utterance structure. Thus 'cover', 'note', 'find' all occur in '—ing', 'We’ll — his path', as well as in '—s' (‘plural’), 'You can have my —'. But 'think' occurs only in the first two types of environment, and 'child' only in the last two. In general, practically all morphemes which occur in '—ing' also occur in many environments like 'We’ll — his path'. Similarly, almost all morphemes which occur in '—s' (‘plural’) occur also in many environments like 'You can have my —'.
- Harris (1946), a pag.164