How do we know that a sequence of words is a compound? Surprisingly, it’s not that easy to come up with a single criterion that works in all cases. Spelling is no help at all; in English there is no fixed way to spell a compound word. Some, like greenhouse, are written as one word, others like dog bed, as two words, and still others, like producer-director are written with a hyphen between the two bases.

A better criterion is stress; compounds in English are often stressed on their first or left-hand base, whereas phrases typically receive stress on the right. Compare, for example, a greenhouse, which is the place where plants are grown, to a green house, that is, a house that’s painted green. But it’s not always the case that compounds are stressed on the left. For example, most people pronounce apple pie with stress on the second base, but apple cake with stress on the left one. Yet we have the feeling that both are compounds; it seems illogical to consider one a compound and not the other.

There is, however, one test for identifying compounds that is fairly reliable: we can test for whether a sequence of bases is a compound by seeing if a modifying word can be inserted between the two bases and still have the sequence make sense. If a modifying word cannot sensibly be inserted, the sequence of two words is a compound. This test confirms that both apple pie and apple cake are compounds, in spite of their differing stress. In neither case can we insert a modifier like delicious between the two stems; *apple delicious pie and *apple delicious cake are equally peculiar!