Some classes may turn out to be included only in a low-numbered sequence symbol: e.g. 'Vn' occurs only in 'V¹Vn=N¹' ('abolition' substitutable for 'bread'); others are included only in a high-numbered symbol: e.g. 'Vυ' in 'V³Vυ=V⁴' ('walked', or 'tried to escape' substitutable for 'walk'). We may say that the 'V³' domain of '–ed' is greater (in number of possible substitutes and often in number of successive morpheme places) than the 'N¹' domain of 'Vn'. Similarly, 'P' occurs only with 'N⁴' and 'N⁵', and may therefore be said to apply to a complete noun-phrase. Some classes occur frequently as the resultants of sequences (i.e. on the right hand side of the equation) and rise to fairly high numbers (e.g. 'N⁵'); others occur rarely as resultants, and require no numbering (e.g. 'D', all of whose equivalents may be substituted for each other.).
- Harris (1951), a pag.277