I may have the spellings wrong, but these are words from greek used in rhetoric. Are they the same thing? Both seem to be defined as using the part of whole to stand in for an unnamed thing. Like: the pen is mightier than the sword. Pen refers to thoughts/writing/ideas and sword to brute force/armed force/military.
I looked the words up on some good rhetoric web sites, but didn’t get the difference. Is there one?
Thanks! 
Did you look at a dictionary?
A metonymy is when one word that is closely associated with another word is used. That would be the example you cited.
A synechdoche is when a part of something is used to represent a whole, or when a whole represents a part. (e.g. All hands on deck. Five souls were aboard the sinking ship.) The difference is that one is a part of the other. In a metonymy, the words are just closely associated.
As I said, I did look. Did you read my post?
Thanks, however, your examples made it clearer. I didn’t see before the distinction between “just associated” and “parts for whole.” Afterall, a pen is part of writing and sword part of military, but now I get the idea of it.
Synecdoche (sin-ekk-doh-kay): using the part in place of, or to represent, the whole.
I live in England, and people often make the mistake of referring to England when they mean all of Great Britain.
Metonymy (met-on-a-me): using one term in place of, or to represent, another with which it is associated - most commonly to identify a person by something associated with their role, status or office. Example: referring to “the Crown” when it is understood that you are referring to the person who is the reigning monarch. Another common example is to refer to the “chair” at a meeting, when you mean the person who is in charge of, or chairing, the meeting.
…just stopped by to learn the meanings of the two words. Thanx, I am two words smarter today.