A frequent type of limitation is that in which members of one subclass of a general class occur with each other, but do not occur with members of another sub-class of the same general class. Thus in the English general class 'N' we have 'book', 'artist', 'author', 'cow', 'bull', 'king', 'queen', etc. In the sequence class 'N⁴' […] we have 'this old-fashioned artist', 'our cow', 'he', 'she', 'I', etc. Of all these, certain members occur together in the two 'N' positions of 'N Vь⁴ N⁵'. One group of members which occur together in these 'N' positions may be called 'Nք' and contains 'she', 'cow', 'queen', etc.: 'She’s a good cow', 'She will remain as queen', 'The cow is the queen of farm animals'. Another group of members which occur together in these 'N' positions may be called 'Nա' and contains 'he', 'bull', 'king', etc.: 'He’s a fine bull', 'He’ll rermain as king', 'The bull is the king of farm animals'. Members of 'Nք' hardly ever occur with members of 'Nա' in the environment 'NV ьN'. If the first 'N' of 'NV ьN' is 'he' or 'the bull', the second 'N' will not be 'she' or 'queen': our corpus will not contain 'What breed of bull is she?'
- Harris (1951), a pag.303