The paper begins with a discussion of several recently proposed analyses of nominal compounds in English. It is then suggested that the relations which may appropriately underlie nominal compounds of the type Noun + Noun can best be defined negatively, i.e. by listing those relations between two nouns which cannot underlie compounds rather than those which can. It is further argued that in order for compound formation to take place the relation between the nouns in question must be "appropriately classificatory." A brief examination of abstract compounds in English and German leads to the conclusion that there are probably fewer systematic restrictions valid across languages on the formation of abstract than of concrete nominal compounds. The appendix to the paper contains a brief examination of English nominal compounds with a primary + tertiary stress pattern (e.g., bookstore) and nominal phrases with a secondary + primary stress pattern (e.g., morning coffee). (Author)