The chief novelty in the procedure which is offered here is the extension of substitution classes to include sequences of morphemes, not merely single morphemes. We now ask not only if 'A' and 'B' each occur in the environment 'C — D' but also if 'AE' together, or 'FGH', also occur in that environment. If they do, then 'A', 'B', 'AE', 'FGH' are all substitutable for each other. We may say that they are all members of one substitution class, which is now not merely a class of morphemes but a class of morpheme sequences. The single morphemes in the class are merely the special cases where the sequence consists of one. Thus we note that in 'Please put the book away' we can substitute for 'book' not only other single morphemes like 'bottle' or 'brandy' but also sequences of two and more morphemes like 'books', 'bank-book', 'brandy bottle', 'bottle of brandy', 'silly green get-up'. These sequences differ in various respects: in 'brandy bottle' each of the component morphemes could have been substituted singly for 'book'; in 'books' only the first could; in 'silly green get-up' no one of the morphemes could (in most utterances) have alone been substituted for 'book'.
- Harris (1946), a pag.165