A metaphor is a literary device in which something, be it an object or something more abstract, is said to be something else. In an extended metaphor, also known as a conceit, this comparison is continued for quite some time in the particular work of literature. In some cases, the extended metaphor will be used throughout a single sentence or paragraph while in other cases it is continued throughout the entire piece. Metaphors cause the reader to look at a given subject in a different way. In some cases, a concrete representation for an abstract concept is given while in other cases something concrete is equated to something deeper and more abstract.
While there are many different ways to craft an extended metaphor, many writers start with a single main comparison before discussing how other ideas fit into that comparison. A main idea in an extended metaphor could, for instance, compare a man to a tree. Further discussion could give more specific metaphorical meaning to the branches, trunk, root, and other elements of the tree. A regular metaphor might stop after comparing the man to a tree, but a conceit needs to extend the comparison. An effective extended metaphor can greatly clarify the ideas that the writer wishes to communicate while a poor metaphor may simply confuse the reader by introducing too many new comparisons.
A writer can use an extended metaphor in many different ways. In some cases, an entire story will be told metaphorically, often to illustrate a point or to teach a moral lesson of some form. In other cases, such metaphors are used only to describe one particular action, object, or character with greater depth. Metaphors, extended and otherwise, are commonly used in all forms of literature, but are particularly common in poetry and in fiction. Entire poems based on a single metaphor, for example, are quite common.
Extended metaphors can be used effectively for a wide variety of reasons. In many cases, they are simply used as a means of description when concrete terms are not sufficient or when the writer wants to add rhetorical flourish to a piece. The literary technique is commonly used in philosophical and political writing as well. In many cases, the concepts addressed in such subjects can be extremely difficult to grasp without a description based in more common ideas, so metaphors must be used for clarification. Metaphors are sometimes even used in science, as some topics in chemistry, physics, and biology can be more easily explained in terms of easily-visualized phenomena rather than through precise description of the actual processes.