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Analogy
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Analogy

An analogy, loosely, is a comparison between two things. Analogies are often used to introduce new concepts, by comparing them to something familiar, or to support political and philosophical arguments.

In anatomy, two anatomical structures are considered to be analogous when they serve similar functions but are not evolutionarily related, such as the legs of vertebrates and the legs of insects. Analogous structures are the result of convergent evolution and should be contrasted with homologous structures.

In linguistics, analogy is a process that reduces forms perceived as irregular by remaking them in the shape of more common forms that are governed by rules. For example, the English verb once had the preterite holp and the past participle holpen. These obsolete forms have been discarded and replaced by helped by the power of analogy.

The SAT college entrance test includes "analogy" questions that ask for comparisons between analogies: A is to B as C is to what? For example:

Hand is to palm as foot is to ____?
These questions are usually given in the format:
HAND : PALM : : FOOT : ____

Certain aspects of analogy have a precise formulation in mathematics as the concept of isomorphism.