Synecdoche:
Sometimes the material an item is made of can be used as
synecdoche in place of the actual object. When a sword is referred to as
'steel,' for instance, this is synecdoche, since the entire sword is probably
not made of steel. Moreover, the sword could be made of another metal
altogether, but the historical connection between 'steel' and 'sword' is
powerful enough to make it synecdoche nonetheless.
Likewise, if someone asks, 'Are you wearing Kevlar?' you
might know from watching enough action movies that this is synecdoche for a
bulletproof vest, while using 'plastic' at the grocery store means putting the
bill on your credit card because credit cards are made of plastic. As with
materials, containers can sometimes come to refer to the objects they contain -
another form of synecdoche. As in, 'Nazie drank the cup,' which doesn't
literally mean that Nazie swallowed a small cup, but rather that she drank the
contents of the cup. Likewise, 'The bartender is giving away the bar,' means
that he or she is giving out too many free drinks, which is the stuff the bar
contains.
Metonymy
Metonymy is when a thing refers to something else that it's
closely associated with, but unlike synecdoche, the part does not have to refer
to the whole, or vice versa. Remember when we talked about how 'wheels' was
synecdoche for 'car?' Here's the metonym version of the same:
'It was the town's mechanic, not the rich lawyer, who had
the nicest ride.'
'Ride' here is a metonym for 'car' because riding is
something you do in an automobile, but the 'ride' is not a part of the
automobile and therefore does not qualify as synecdoche.