Metonymy is a linguistic device by which one thing is referenced by directly referring to something else that is associated with it. For example, citizens of the US often use the word “Washington” in reference to Washington, D.C., to refer to the US government since the leaders of the federal government work in that city. Similarly, in many countries and time periods that have a ruling class of monarchy, the term “crown” is often used to refer to the actual ruler. Metonymy is somewhat related to and taught with other devices such as metaphor and synecdoche, though they are not the same.
The basic idea behind the usage of metonymy is that people create associations between a particular object and a related object. In the phrase “the pen is mightier than the sword,” for example, there are two instances of metonymy at work. “The pen” does not literally refer to a writing instrument, but instead refers to the process of writing and the expression of ideas, while “the sword” again does not mean a literal weapon, but instead refers to a military group or armed action. Someone who hears this type of phrase is typically able to understand what is meant, since these associations are quite common within a particular culture or society.
Metonymy is often related with metaphor, since they can function in somewhat similar ways, though they serve very different purposes. Both devices function by taking advantage of similarities between two things, but metonymy makes this connection through a relationship that is already established between the two things. The use of the word “crown” to indicate royalty utilizes the fact that there is already an explicit connection between royal traditions and the wearing of a crown. A metaphor like “her beauty was an arrow” is not utilizing an already established relationship; it can change in meaning depending on what follows it, such as “that struck me in the chest” or “that she was quick to fire, but lacked precision.”
Synecdoche is also a linguistic device often related to metonymy. The primary difference, however, is that synecdoche specifically uses something that is a part of something else to refer to it. For example, someone who refers to his or her car by saying “check out my wheels” is using synecdoche. In this usage, he or she does not typically mean for someone to just look at the wheels of the car, but uses that one part of the car to refer to the entire object. Some people consider synecdoche to be a specific type of metonymy, though this is not universal and many instructors teach them as separate devices.