"

 When one stands for the rest 

In restaurants, I often hear people order a Coke, even if the restaurant serves Pepsi.  It's been my observation that people say ""Coke"" to refer to any kind of cola, even using the brandname in preference to the beverage name.  For example, the drink is called a ""rum and coke,"" not a ""rum and cola.""  Is this a metonym?"