Talk about la même chose!

The Oxford Dictionary of English tells me that 'synechoche' is 'a figure of speech in which a part is made to represent the whole or visa vera; while 'metonymy' is the subtitution of the name of an adjunct for that of the thing meant.

Well, does this simply mean that 'metonymy' is a certain kind of 'synechoche' -- the part for the whole?

Surely these are murky waters?