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From the definitions of these two types of figures of speech (tropes, if you will), I have always understand them to mean the same thing. Essentially, that is the usage of either a specific attribute or a generalisation of a thing to refer to that thing.
Could anyone perhaps qualify the difference between a synecdoche and metonym; is one a sub-category of another, are these essentially equivalent, or are they mutually exclusive in some subtle way?
They’re not the same thing, though metonymy is often interpreted so widely that synecdoche can be regarded as a special case of it. You use synechdoche when you speak of a part of something but mean the whole thing. Example: "All hands on deck" is an example in which 'hands' is used to mean 'people'. You can also reverse the whole and the part, so using a word for something when you only mean part of it. Metonymy is similar, but uses something more generally or loosely associated with a concept to stand in for it. In other words, is the substitution of one word for another object or idea which it suggests or the substitution of the name of an attribute or feature for the name of the thing itself. Example: "The pen is mightier than the sword," meaning literary power is superior to military force. In the case of the sentence “Get away from me or I’ll hit you with the milk” the choice is difficult. That’s a case of metonymy all right, but you can argue it’s also synecdoche, because milk is an essential component part of a bottle of milk, not just something associated with it. I hope you'll find this explanation useful.
Using the definitions from silva rhetoricae
Reference to something or someone by naming one of its attributes.
A whole is represented by naming one of its parts (genus named for species), or vice versa (species named for genus).
It appears that there is little room for misinterpretation, however in the definition of metonymy the 'attribute' is understood very flexibly and it expands the definition to 'a quality or feature of someone or something'.
Observe the example and the explanation of metonymy (from silva rhetoricae):
The pen is mightier than the sword. The pen is an attribute of thoughts that are written with a pen; the sword is an attribute of military action.
This is similar to wikipedia's definition (of metonymy) and under this definition synecdoche can be seen as a special case of metonymy.
