A 'GREENhouse' is a glass building for plants. A 'green HOUSE' is simply a house painted green. Same words, different meanings, and the only clue in speech is where you put the stress. This is one of the most powerful rules for sounding clear.
The Rule
In a compound noun (two words that act as a single noun), stress the first word: GREENhouse, BLACKboard, BOOKstore. In an ordinary adjective + noun phrase, stress the second word: a green HOUSE, a black BOARD. The stress tells the listener whether you mean one fixed thing or a described thing.
Compound nouns: stress the FIRST word
Phrases and contrasts: stress the SECOND word
Why This Matters for Meaning
Stress is not decoration; it carries meaning. 'A WHITE House' is any house painted white, but 'the White HOUSE'... no, the residence is 'the WHITE House' with first-word stress, because it is a compound name. Compare 'a hot DOG' (a dog that is warm) with 'a HOT dog' (the food). Getting the stress right prevents real confusion.
Exceptions to Watch
Not every two-word noun is first-stressed. Compounds built around materials or certain patterns keep second-word stress: 'apple PIE', 'chocolate CAKE', 'paper BAG'. Many street names also stress the second word ('Fifth AVENUE') while others stress the first ('MAIN Street'). When in doubt, learn the common ones as fixed phrases.
Practice Tip
Say pairs aloud and exaggerate: GREENhouse / green HOUSE, DARKroom / dark ROOM. For a related pattern, see our guide to street name stress.