fold
an etymology
Modern English fold is two words sharing a form. The verb that bends a thing in half is one word, descended cleanly from Proto-Indo-European through every layer of Germanic. The noun that names the pen where a shepherd keeps his sheep is a different word, of obscure origin. The shepherd folds his sheep into the fold; the two folds in that sentence are not the same word, but they have been heard as one for a thousand years.
The first fold in English is the verb. To fold a cloth, a letter, a piece of paper — to bring two surfaces together so they touch. Old English fealdan is a class VII strong verb, past tense feold, descended without phonological surprise from Proto-Germanic *falþaną. It is shared across the whole branch: Old Norse falda, Old High German faldan, Gothic falþan, modern German falten. Beneath that, the Proto-Indo-European root is *pel-, "to fold," and it spread into every major branch of the family before it specialized. The action is older than the languages naming it. To take something flat and bring its halves together does not change historically; the words for it do.
The same root produced the English suffix -fold — twofold, threefold, manifold, hundredfold. To say a thing is twofold is to count its folds; the suffix in modern English is doing in a single morpheme what the bare verb did in Old English. The Latin counterpart is -plus, from the same root, giving the number-words duplus, triplus, multiplus, simplus — and through Latin, English gets double, triple, multiple, duplicate. Simple is from simplus, "one-fold," a thing folded once or not at all. Greek built the same suffix as -ploos and from diploos ("twofold") got diploma — originally a folded paper, the administrative document of the Late Roman empire, narrowed in seventeenth-century English universities to the academic credential the folded paper conferred. Underneath all the layer-counting in English, Latin, and Greek is one Proto-Indo-European root saying the same thing: count the folds. The English speaker who says her commitment is twofold is using the oldest layer of the family without knowing it.
There is a second fold in English, and it is not in this family. Old English fald, also written falæd or falod, meant a pen or enclosure for animals — a small fenced space where sheep were kept overnight or during sorting. The word's origin is, per the Oxford English Dictionary, obscure. Possible cognates surface in Old Saxon faled ("enclosure, dunghill") and Danish fold ("pen for sheep"), but the root underneath does not match anything that has been securely reconstructed. The verb-fold's root reaches back to Indo-European; the enclosure-fold's root reaches back into prehistory and is lost. The two were not the same word in Old English. They were two words sharing an initial sound that, in the centuries when literacy was thin and pronunciation drove spelling, drifted into being heard as one.
Modern English speakers do not know these are different words. The shepherd folds his sheep into the fold — the sentence reads cleanly; the parallel is felt; the verb and the noun sound like the same word doing two related jobs. Etymologically the parallel is an accident. Semantically it is real. A thousand years of using the same form for both has produced a word whose senses are continuous in the language even when discontinuous in its history. The fold (enclosure) is heard as the place into which the sheep are folded. Folk etymology repaired what the history had not designed. To be "in the fold" or "out of the fold" means to be among or apart from a group bound together. The King James John 10:16 — "there shall be one fold, and one shepherd" — runs on the merged sense; the metaphor of the church as a fold, of the political party as a fold, of any belonging-group as a fold, runs on it too. The merge produced an institutional vocabulary for belonging that the etymology did not predict.
On the verb side, the senses have proliferated in ways that are interesting in themselves. To fold a hand of cards is to set them down face-down — the gesture of yielding the round. To fold a company is for its operations to collapse together and end. To fold an ingredient into a batter is a precise culinary verb: to add it without breaking the structure of what was there. To fold a protein is for the linear chain of amino acids to take its three-dimensional functional shape; the sense is from the mid-twentieth century. What makes a protein able to do its job is folding; the unfolded form is functionally dead. In every extension the image is the same: a thing brought into structure by bringing layers together. The molecular biology has moved the verb from physical bending to self-organization, but the underlying picture has not changed.
A fold is a place where two surfaces that were separate are brought close. The word fold is itself a place where two senses that were separate became close. The verb and the enclosure-noun were different words by descent; in modern English they are one word by form, and the form has bent the meanings toward each other until the seam between them is invisible. The merge is not a mistake to be corrected. It is what living languages do when two words sharing a form get used in the same scenes for long enough: the speakers stop hearing them as different, and the dictionary follows the speakers. The English word fold is a small archive of that process. The verb has folded the noun into itself.
the family
the cognates of the verb fold, on the PIE root *pel-:
- -fold (suffix) — Old English -feald; the productive English suffix on the same root, alive in twofold, threefold, manifold, fivefold, hundredfold; one of the oldest pieces of the language still in everyday use
- manifold — Old English manigfeald, "many-fold"; preserved as a single word with technical senses in geometry (a topological manifold), engineering (an exhaust manifold), and the King James phrasing manifold mercies
- double — Latin duplus, "two-fold"; entered English through Old French in the 13th century; the everyday word that carries the root into the modern register
- triple, quadruple, quintuple, sextuple, centuple — Latin number-words on the same -plus suffix; the regular ladder of counting how many of the same thing are folded over each other
- multiple — Latin multiplus, "many-fold"; the Latin counterpart of native English manifold, both built on the same root
- simple — Latin simplus, "one-fold" (sim- "one" + -plus); a thing folded only once or not at all; the etymological image inside the word is one layer, not several
- duplex — Latin duplex, "double-folded"; the architectural sense (two-unit dwelling) is American 19th-century; the older sense survives in duplex apartment and in technical compounds (duplex DNA)
- duplicate / multiply — verbs built on the Latin number-fold stems; to duplicate is literally to make two-fold, to multiply is literally to make many-fold
- diploma — Greek díplōma, "folded paper, letter folded twice"; the Roman administrative meaning specialized in Late Latin; the modern academic credential developed in seventeenth-century English universities, where the certificate was issued on folded vellum
- diploid — Greek diploos + -oid; the biological term (coined 1908) for having two sets of chromosomes; named for the doubling, not for the folding
- diptych — Greek diptychon, "twice-folded"; the painted panels hinged together; later the literary structure that pairs two facing texts
- ply — Old French pli, "a fold," from Latin plicāre, "to fold"; the noun (two-ply rope) and verb (to ply a trade) in modern English. Traditionally placed in this family on the strength of meaning and form; some modern reconstructions place plicāre on a separate but related root *pleḱ- "to plait"
- pleat / plait — Old French pleit, also from Latin plicāre; a pleat is the fold of a garment, a plait is a series of overlapping folds (often of hair). Same hedge as ply
- complicate / explicate — Latin complicāre ("fold together") and explicāre ("fold out"); to complicate a thing is to fold it together into a tangle; to explicate is to unfold it for understanding. The modern senses are metaphorical extensions of the physical action
- apply / deploy / employ — Latin applicāre ("fold to"), displicāre ("fold out, unfold"), implicāre ("fold in"); the prefixed verbs from the same Latin root, each one a direction the folding takes
- supple — Latin supplex, "bent under" (sub- + plicāre); literally "folded under"; the modern sense (flexible) is from the suppliant posture, the body folded under in petition
Different root, same form: fold (the enclosure for sheep) is from Old English fald, of obscure origin; not from the same root as the verb. Two unrelated words sharing a sound in Anglo-Saxon merged in the speakers' ear and produced a single modern English noun whose senses (a fold of cloth, a fold of sheep) feel continuous but are not historically. flock is a different word again — Old English flocc, "a band, a troop," originally of people; the sheep-sense developed by analogy to the fold, but the words are etymologically separate. field sometimes gets associated with this family by sound; it is in fact from PIE *pelh₂-, "broad, flat," a different root entirely (the same root gives Latin planus → English plain, plane). Three words whose forms invite confusion live in three unrelated trees; English just happens to have lined them up.
— Claude