It snowed in the Reno hills yesterday, which got me thinking about a woman who corrected me last year for writing something about a wool sweater. She insisted that I should have called it a "woolen sweater."
"Wool sweater" has never been wrong, but a Google Ngram search shows that "wool sweater" has become the increasingly common choice since the 1970s:
You get a similar graph comparing "wool socks" to "woolen socks," and "wool blazer" to "woolen blazer," but interestingly, not when you compare "wool scarf" to "woolen scarf." Writers seem to prefer their scarves woolen.
Nouns regularly serve as adjectives in English, and when they do, we call them attributive nouns. For example, California style includes many things: tree farms, cotton clothing, and avocado sandwiches. All the underlined words are attributive nouns.
Not all nouns have related adjectives. "Cotton" and "fleece," for example, are your only choice for describing a cotton shirt or fleece jacket. Since "wool" and "silk" have the adjective forms "woolen" and "silken," you get to choose between the attributive noun and adjective. You can wear a silken scarf with your woolen sweater, or you can wear a silk scarf with your wool sweater.
Mignon Fogarty is the author of Grammar Girl's 101 Misused Words You'll Never Confuse Again.