This from Ask Figaro:
I saw this note on a high school student’s locker: “Caution: Blonde trapped in a brunette’s body.”
What would you call this? This is the one where a part stands for the whole, right? Is that synecdoche?
Close but no metaphorical cigar! That blonde is a metonymy, which transfers one trait onto another. A synecdoche swaps the part for the whole, while a metonymy takes a characteristic and makes it represent the thing it characterizes. Tricky stuff, I know. To learn more, go to our sister site and read this extremely short detective story:http://www.wordhero.org/story.