Etymology
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*reig- 

Proto-Indo-European root meaning "stretch; be stretched; be stiff" 

It forms all or part of: reach; rigid; rigidity; rigor.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rjyati "he stretches himself," riag "torture" (by racking); Greek oregein "to reach, extend;" Latin rigidus "hard, stiff, rough, severe," rigēre "be stiff;" Lithuanian raižytis "to stretch oneself;" Old Irish ringid "torture," rigim "I stretch;" Middle High German ric "band, string;" Old High German reihhon, Old English ræcan, ræcean "to reach, achieve," on notion of "to stretch out." 

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*stere- 
*sterə-, also *ster-, Proto-Indo-European root meaning "to spread."

It forms all or part of: consternate; consternation; construct; construction; destroy; destruction; industry; instruct; instruction; instrument; obstruct; obstruction; perestroika; prostrate; sternum; sternocleidomastoid; strain (n.2) "race, stock, line;" stratagem; strategy; strath; strato-; stratocracy; stratography; stratosphere; stratum; stratus; straw; stray; street; strew; stroma; structure; substrate; substratum; substructure.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit strnoti "strews, throws down;" Avestan star- "to spread out, stretch out;" Greek stronymi "strew," stroma "bedding, mattress," sternon "breast, breastbone;" Latin sternere "to stretch, extend;" Old Church Slavonic stira, streti "spread," strana "area, region, country;" Russian stroji "order;" Gothic straujan, Old High German strouwen, Old English streowian "to sprinkle, strew;" Old English streon "strain," streaw "straw, that which is scattered;" Old High German stirna "forehead," strala "arrow, lightning bolt;" Old Irish fo-sernaim "spread out," srath "a wide river valley;" Welsh srat "plain."
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*(s)pen- 

Proto-Indo-European root meaning "to draw, stretch, spin."

It forms all or part of: append; appendix; avoirdupois; compendium; compensate; compensation; counterpoise; depend; dispense; equipoise; expend; expense; expensive; hydroponics; impend; painter (n.2) "rope or chain that holds an anchor to a ship's side;" pansy; penchant; pend; pendant; pendentive; pending; pendular; pendulous; pendulum; pension; pensive; penthouse; perpendicular; peso; poise; ponder; ponderous; pound (n.1) "measure of weight;" prepend; prepense; preponderate; propensity; recompense; span (n.1) "distance between two objects;" span (n.2) "two animals driven together;" spangle; spanner; spend; spider; spin; spindle; spinner; spinster; stipend; suspend; suspension.

It is the hypothetical source of/evidence for its existence is provided by: Latin pendere "to hang, to cause to hang," pondus "weight" (perhaps the notion is the weight of a thing measured by how much it stretches a cord), pensare "to weigh, consider;" Greek ponos "toil," ponein "to toil;" Lithuanian spendžiu, spęsti "lay a snare;" Old Church Slavonic peti "stretch, strain," pato "fetter," pina "I span;" Old English spinnan "to spin," spannan "to join, fasten; stretch, span;" Armenian henum "I weave;" Greek patos "garment," literally "that which is spun;" Lithuanian pinu "I plait, braid," spandau "I spin;" Middle Welsh cy-ffiniden "spider;" Old English spinnan "draw out and twist fibers into thread," spiðra "spider," literally "spinner."

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*ten- 
Proto-Indo-European root meaning "to stretch," with derivatives meaning "something stretched, a string; thin."

It forms all or part of: abstain; abstention; abstinence; abstinent; atelectasis; attend; attenuate; attenuation; baritone; catatonia; catatonic; contain; contend; continue; detain; detente; detention; diatonic; distend; entertain; extend; extenuate; hypotenuse; hypotonia; intend; intone (v.1) "to sing, chant;" isotonic; lieutenant; locum-tenens; maintain; monotony; neoteny; obtain; ostensible; peritoneum; pertain; pertinacious; portend; pretend; rein; retain; retinue; sitar; subtend; sustain; tantra; telangiectasia; temple (n.1) "building for worship;" temple (n.2) "flattened area on either side of the forehead;" temporal; tenable; tenacious; tenacity; tenant; tend (v.1) "to incline, to move in a certain direction;" tendency; tender (adj.) "soft, easily injured;" tender (v.) "to offer formally;" tendon; tendril; tenement; tenesmus; tenet; tennis; tenon; tenor; tense (adj.) "stretched tight;" tensile; tension; tensor; tent (n.) "portable shelter;" tenterhooks; tenuous; tenure; tetanus; thin; tone; tonic.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tantram "loom," tanoti "stretches, lasts," tanuh "thin," literally "stretched out;" Persian tar "string;" Lithuanian tankus "compact," i.e. "tightened;" Greek teinein "to stretch," tasis "a stretching, tension," tenos "sinew," tetanos "stiff, rigid," tonos "string," hence "sound, pitch;" Latin tenere "to hold, grasp, keep, have possession, maintain," tendere "to stretch," tenuis "thin, rare, fine;" Old Church Slavonic tento "cord;" Old English þynne "thin."
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*reg- 

Proto-Indo-European root meaning "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule."

It forms all or part of: abrogate; address; adroit; Alaric; alert; anorectic; anorexia; arrogant; arrogate; bishopric; correct; corvee; derecho; derogate; derogatory; Dietrich; direct; dress; eldritch; erect; ergo; Eric; Frederick; Henry; incorrigible; interregnum; interrogate; maharajah; Maratha; prerogative; prorogue; rack (n.1) "frame with bars;" rail (n.1) "horizontal bar passing from one post or support to another;" Raj; rajah; rake (n.1) "toothed tool for drawing or scraping things together;" rake (n.2) "debauchee; idle, dissolute person;" rakish; rank (adj.) "corrupt, loathsome, foul;" real (n.) "small Spanish silver coin;" realm; reck; reckless; reckon; rectangle; rectify; rectilinear; rectitude; recto; recto-; rector; rectum; regal; regent; regicide; regime; regimen; regiment; region; regular; regulate; Regulus; Reich; reign; resurgent; rex; rich; right; Risorgimento; rogation; royal; rule; sord; source; subrogate; subrogation; surge; surrogate; viceroy.

It is the hypothetical source of/evidence for its existence is provided by:

Sanskrit raj- "a king, a leader," rjyati "he stretches himself," riag "torture" (by racking); Avestan razeyeiti "directs," raštva- "directed, arranged, straight;" Persian rahst "right, correct;" Latin regere "to rule, direct, lead, govern," rex (genitive regis) "king," rectus "right, correct;" Greek oregein "to reach, extend;" Old Irish ri, Gaelic righ "a king," Gaulish -rix "a king" (in personal names, such as Vircingetorix), Old Irish rigim "to stretch out;" Gothic reiks "a leader," raihts "straight, right;" Lithuanian raižytis "to stretch oneself;" Old English rice "kingdom," -ric "king," rice "rich, powerful," riht "correct;" Gothic raihts, Old High German recht, Old Swedish reht, Old Norse rettr "correct."

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*gleubh- 

Proto-Indo-European root meaning "to tear apart, cleave." 

It forms all or part of: cleave (v.1) "to split, part or divide by force;" cleft; clever; clevis; clove (n.2)  "slice of garlic;" glyptodon; hieroglyphic; petroglyph.

It is the hypothetical source of/evidence for its existence is provided by: Greek glyphe "a carving," glyphein "to hollow out, cut out with a knife, engrave, carve;" Latin glubere "to peel, shell, strip;" Old High German klioban, Old English cleofan, Old Norse kljufa "to cleave," Old Norse klofi, Middle Dutch clove "a cleft."  

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*kele- (2)
*kelə-, Proto-Indo-European root meaning "to shout." Perhaps imitative.

It forms all or part of: acclaim; acclamation; Aufklarung; calendar; chiaroscuro; claim; Claire; clairvoyance; clairvoyant; clamor; Clara; claret; clarify; clarinet; clarion; clarity; class; clear; cledonism; conciliate; conciliation; council; declaim; declare; disclaim; ecclesiastic; eclair; exclaim; glair; hale (v.); halyard; intercalate; haul; keelhaul; low (v.); nomenclature; paraclete; proclaim; reclaim; reconcile.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usakala "cock," literally "dawn-calling;" Latin calare "to announce solemnly, call out," clamare "to cry out, shout, proclaim;" Middle Irish cailech "cock;" Greek kalein "to call," kelados "noise," kledon "report, fame;" Old High German halan "to call;" Old English hlowan "to low, make a noise like a cow;" Lithuanian kalba "language."
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*agro- 
Proto-Indo-European root meaning "field;" probably a derivative of root *ag- "to drive, draw out or forth, move."

It forms all or part of: acorn; acre; agrarian; agriculture; agriology; agro-; agronomy; onager; peregrinate; peregrination; peregrine; pilgrim; stavesacre.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ajras "plain, open country," Greek agros "field," Latin ager (genitive agri) "a field," Gothic akrs, Old English æcer "field."
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*bhag- 
Proto-Indo-European root meaning "to share out, apportion; to get a share."

It forms all or part of: aphagia; Bhagavad-Gita; baksheesh; esophagus; nebbish; pagoda; -phage; phago-; -phagous; porgy; sarcophagus.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhajati "assigns, allots, apportions, enjoys, loves," bhagah "allotter, distributor, master, lord," bhaksati "eats, drinks, enjoys;" Persian bakhshidan "to give;" Greek phagein "to eat," literally "to have a share of food;" Old Church Slavonic bogatu "rich."
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*pete- 
*petə-, Proto-Indo-European root meaning "to spread."

It forms all or part of: compass; El Paso; expand; expanse; expansion; expansive; fathom; pace (n.); paella; pan (n.); pandiculation; pas; pass; passe; passim; passacaglia; passage; passenger; passport; paten; patent; patina; petal; spandrel; spawn.

It is the hypothetical source of/evidence for its existence is provided by: Greek petannynai "to spread out," petalon "a leaf," patane "plate, dish;" Old Norse faðmr "embrace, bosom," Old English fæðm "embrace, bosom, fathom," Old Saxon fathmos "the outstretched arms."
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