1. I nternal isoglosses . A study of compound words—words containing more than one morph other than pitch and inflectional ending—shows a definite cleavage of morphs into two classes, here called class S (for Sino-Japanese, the historical origin of the class) and class Y (for Yamato, or native Japanese, the presumed origin of most members of the class). There are numerous hybrid compounds, to be sure; but on the basis of selectivity within immediate constituents which contain only two morphs, for the overwhelming majority of cases, each morph and morph group may be placed clearly in one of the two classes. There is evidence, less obvious, for further selectivity within these classes, pointing to subclasses (historically, the chronologically different layers of Sino-Japanese), but the difficulty of abstracting and interpreting this evidence descriptively probably outweighs the usefulness of such subclasses, even if they could be clearly delineated. There is also a class of morphs found only in European loanwords; these are discussed in §17.5.
A Mon, study studied this question.