A merism is a figure of speech in which something is described by enumerating several of its different traits or components. Merisms often indicate completeness. They are also usually conventional phrases, reused in the same way by most English speakers. Examples of common English merisms include "hook, line, and sinker" and "high and low."
A merism serves to define a whole by describing some or all of its parts. There are two primary methods of accomplishing this. The first is to describe two contrasting extremes. For example, the merism "young and old" describes the whole population; if someone claims that a product will appeal "to young and old," he is making the claim that the product will appeal to everyone. Similarly, if someone wishes to convey that she has searched an entire area, she may use the expression "searched high and low," which uses two contrasting extremes to define the whole.
The second common type of merism is one in which a number of the parts of a thing, although not necessarily all, are used to denote the whole. A classic example of this is the expression "lock, stock, and barrel," which originally referred to the parts of a gun. It now refers to the whole of any object. A similar expression, "hook, line, and sinker" refers to the parts of a fishing line. "He swallowed it hook, line, and sinker" means "he swallowed it completely."
Merisms are stock phrases in English, and are almost always repeated in an identical format. To return to the example of "lock, stock, and barrel," it would be unheard-of for an English speaker to say "stock, lock, and barrel" or "barrel, stock, and lock." Similarly, people search "high and low," not "low and high," and it is far more common for a crowd to be addressed as "ladies and gentlemen" than "gentlemen and ladies."
Merisms are a common feature of legal writing, where they often originate from terms which previously had distinct meanings but no longer do. Examples of legal merisms include "last will and testament." Merisms are also common rhetorical devices in the Bible, in which, for example, the phrase "the heavens and the earth" signifies all of creation.
The word "merism" comes from a Greek root, "merismos," which means "dividing" or "partition." As a result, the word has an alternative meaning. In biology, merism describes the repetition of similar parts of an organism, such as the repeating pattern of scales on fish. Classifying animals by counting these repeating features is called meristics.