fold

Modern English fold is two words sharing a form. The verb that bends a thing in half — fold a letter, fold a cloth — is one word, descended cleanly from PIE *pel-, "to fold," through every layer of Germanic, and producing a wide layer-counting family across Indo-European: the English suffix -fold (twofold, manifold), Latin -plus in duplus ("two-fold"), triplus, simplus, giving English double, triple, simple, multiple; Greek -ploos in diploos, whence diploma ("a folded paper"); and the Latin plicāre family that gives ply, complicate, apply, deploy, supple. The noun that names the pen where a shepherd keeps his sheep is a different word, of obscure origin; the Oxford English Dictionary calls it "obscure," and modern reconstructions stop at the door of West Germanic. The two were not one word in Old English. They became one word in modern English by sharing a sound for a thousand years. The shepherd folds his sheep into the fold; etymologically, the two folds in that sentence are different words. The language has stopped knowing.

stack

Modern English

fold

verb — to bend a thing in half; to bring two surfaces together so they touch; (extended) to yield (in poker), to collapse and end (a business), to take a three-dimensional shape (a protein). Noun — a fold of cloth, a fold in a landscape, a fold of sheep. The verb-fold and the enclosure-fold are felt as one word in modern English even though they descend from two unrelated roots.

Middle English

folden / fealden

to bend, to wrap; the verb went from class VII strong (past tense feold) to weak (past tense folded) over this period. The noun fold (sheep-pen) survived alongside the verb, the two written and pronounced closely enough that the speakers began hearing them as one word.

Old English

fealdan · fald / falæd / falod

two different words. Fealdan (verb, "to fold, wrap up, furl") was a class VII strong verb (past tense feold, past participle fealden), inherited from Proto-Germanic without phonological surprise. Fald (noun, "enclosure for animals, pen, stable") was a separate noun, attested also as falæd and falod; its origin is, per the OED, obscure.

Proto-Germanic

*falþaną (verb); fald obscure

*falþaną "to fold" underlies the verb across the whole Germanic branch — Old Norse falda, Old High German faldan, Gothic falþan, German falten, Dutch vouwen. The enclosure-noun shows up in Old Saxon faled ("enclosure, dunghill") and Danish fold ("pen for sheep"), but the root underneath does not securely match anything further back.

PIE

*pel- (verb only)

"to fold." Gives the verb-fold across Germanic and a wide layer-counting family across the rest of Indo-European: Latin -plus in duplus, triplus, multiplus, simplus; Greek -ploos in diploos, whence diploma; Sanskrit puṭa-. The Latin verb plicāre ("to fold") with its modern descendants (ply, complicate, apply, deploy) is 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." The enclosure-noun has no PIE reconstruction.

how it shifted

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 merge is what living languages do when two words sharing a form get used in the same scenes for long enough — the speakers stop hearing the difference, and the dictionary follows the speakers.

The first fold in English is the verb. Old English fealdan, a class VII strong verb (past tense feold), descended without phonological surprise from Proto-Germanic *falþaną and is shared across the whole branch — Old Norse falda, Old High German faldan, Gothic falþan, German falten. Beneath that, PIE *pel-, "to fold," and the root spread into every major branch of the family. The action is older than the languages naming it.

The same root produced the English suffix -fold (twofold, threefold, manifold, hundredfold) — to say a thing is twofold is to count its folds. The Latin counterpart is -plus, from the same root, giving the number-words duplus, triplus, multiplus, simplus — 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 official document of the Late Roman administration, 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 telling you to count the folds.

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.

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. 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. On the verb side, the senses have proliferated: to fold a hand of cards (yielding), to fold a company (collapsing), to fold a protein (self-organizing). In every extension, the image is the same: a thing brought into structure by bringing layers together. 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.

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cousins

-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 PIE root into the modern register
triple / quadruple / quintuple / 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 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
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. 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
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
fold (enclosure for sheep) — different root — 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 feel continuous but are not historically
flock — different root — 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 — different root — PIE *pelh₂-, "broad, flat"; sometimes associated with the fold-family by sound; the same root gives Latin planus → English plain, plane