- "Study Material" vs "Studying Material"
- "Participle Phrase" vs "Participial Phrase"
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The difference between [noun noun] and [adjective noun] is that a [noun noun] form is a word (specifically, a noun) and [adjective noun] is a phrase (an N-bar). You can very often tell them apart by stress, since the word usually has stress on the first part, [nOUn noun], and the phrase usually has stress on the second part, [adjective nOUn].