In rhetoric, a merism is a figure of speech by which a single thing is referred to by a conventional phrase that enumerates several of its parts, or which lists several synonyms for the same thing.
Merisms also figure in a number of familiar English expressions. When we mean to say that someone searched thoroughly, everywhere, we often say that someone searched high and low. The phrase lock, stock, and barrel originally referred to the parts of a gun, by counting off several of its more conspicuous parts; we use it to refer to the whole of anything that has constituent parts.
In biology, a merism is a repetition of similar parts in the structure of an organism (Bateson 1894). Such features are called meristic characters, and the study of such characters is called meristics.
Merisms are conspicuous features of Biblical poetry. For example, in Genesis 1:1, when God creates the heavens and the earth (KJV), the two parts combine to indicate that God created the whole universe. Similarly, in Psalm 139, the psalmist declares that God knows my downsitting and mine uprising; indicating that God knows all that the psalmist does.
Merisms frequently figure in the writing of lawyers, and are a hallmark of legal style. The two parts of the legal merism Last Will and Testament at one time referred to two documents, enforced in two separate courts: the will disposed of a decedent's real property while the testament disposed of chattels. It became customary to combine the instruments in a single dispositive document, and the name has continued long after the doctrines that required its use became obsolete in common law.
A lawyer who writes a will typically includes a residuary clause that disposes of any property not covered by a prior section. The weight of tradition is such that the lawyer writing such a document will often phrase it something like this:
Traditionally, a gift of real property was called a "devise" whereas a gift of other property was a "bequest". Nowadays, the words "bequeath" and "devise" are synonymous in most jurisdictions so that I bequeath the rest of my property to. . . is enough in both law and logic to achieve the same result. The entire phrase is an elaborate merism. Many deeds frequently contain a traditional clause that says that the grantee is to have and to hold the property conveyed; this usage goes back to the days in which the instruments were drawn up in Latin, and is sometimes called a habendam et tenendam clause. The use of legal merisms seldom if ever adds legal effect to the document that contains them, and frequently increases their reading difficulty. However, the weight of tradition and the fear that a deviation from the established formula might have unintended legal consequences makes lawyers reluctant to revise the traditional formulae, and their clients, seeing them, at least draw the satisfaction of knowing that their documents appear to be written by a lawyer.
Some merisms were introduced during the period when Norman French words were being absorbed into English. In order to assure that a document was clear to both Normans and Saxons, it was desirable to use both the Saxon-root and French-root synonyms for important words, to avoid a pretext for someone to claim a misunderstanding.