Etymology
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*medhyo- 
Proto-Indo-European root meaning "middle." Perhaps related to PIE root *me- (2) "to measure."

It forms all or part of: amid; intermediate; mean (adj.2) "occupying a middle or intermediate place;" medal; medial; median; mediate; medieval; mediocre; Mediterranean; medium; meridian; mesic; mesial; meso-; meson; Mesopotamia; Mesozoic; mezzanine; mezzo; mezzotint; mid (prep., adj.); middle; Midgard; midriff; midst; midwife; milieu; minge; mizzen; moiety; mullion.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
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*leig- 

Proto-Indo-European root meaning "to tie, bind." 

It forms all or part of: alloy; ally; colligate; deligate; furl; league (n.1) "alliance;" legato; liable; liaison; lien; lictor; ligand; ligament; ligate; ligation; ligature; oblige; rally (v.1) "bring together;" religion; rely.

It is the hypothetical source of/evidence for its existence is provided by: Latin ligare "to bind;" Albanian lidh "I bind," and possibly Middle Low German lik "band," Middle High German geleich "joint, limb."

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*rei- 
Proto-Indo-European root meaning "to flow, run."

It forms all or part of: derive; ember-days; rennet (n.1); Rhine; rialto; rill; rio; rival; rivulet; run; runnel.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rinati "causes to flow," ritih "stream, course;" Latin rivus "stream;" Old Church Slavonic reka "river;" Middle Irish rian "river, way;" Gothic rinnan "run, flow," rinno "brook;" Middle Low German ride "brook;" Old English riþ "stream;" Old English rinnan, Old Norse rinna "to run," Dutch ril "running stream."
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*kerd- 
Proto-Indo-European root meaning "heart."

It forms all or part of: accord; cardiac; cardio-; concord; core; cordial; courage; credence; credible; credit; credo; credulous; creed; discord; grant; heart; incroyable; megalocardia; miscreant; myocardium; pericarditis; pericardium; quarry (n.1) "what is hunted;" record; recreant; tachycardia.

It is the hypothetical source of/evidence for its existence is provided by: Greek kardia, Latin cor, Armenian sirt, Old Irish cride, Welsh craidd, Hittite kir, Lithuanian širdis, Russian serdce, Old English heorte, German Herz, Gothic hairto, "heart;" Breton kreiz "middle;" Old Church Slavonic sreda "middle."
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*keu- 

also *skeu- Proto-Indo-European root meaning "to see, observe, perceive." 

It forms all or part of: Anschauung; caution; cautious; caveat; kudos; precaution; scavenger; scone; sheen; show.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kavih "wise, sage; seer, poet;" Avestan kauui- "seer, poet, wise man;" Middle Iranian škoh "splendor, majesty;" Latin cautio "care, foresight," cautus "careful, heedful," cavere "beware, take heed;" Greek kydos "glory, fame;" Lithuanian kavoti "tend, safeguard;" Armenian cucanem "I show;" Old Church Slavonic čudo "wonder;" Czech (z)koumati "to perceive, be aware of;" Serbian čuvati "watch, heed;" Old English sceawian "to look at," Middle Dutch schoon "beautiful, bright," properly "showy," Old High German scouwon "to watch."

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*er- (2)
Proto-Indo-European root meaning "earth, ground." It forms all or part of: aardvark; aardwolf; earth; earthen; earthy.

It is the hypothetical source of/evidence for its existence is provided by: Old English eorþe "ground, soil, dirt, dry land," Old Norse jörð, Old High German erda, Gothic airþa; Middle Irish -ert "earth."
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*meg- 
Proto-Indo-European root meaning "great."

It forms all or part of: acromegaly; Almagest; Charlemagne; maestro; magisterial; magistral; magistrate; Magna Carta; magnate; magnitude; magnum; magnanimity; magnanimous; magni-; Magnificat; magnificence; magnificent; magnify; magniloquence; magniloquent; Magnus; maharajah; maharishi; mahatma; Mahayana; Maia; majesty; major; major-domo; majority; majuscule; master; maxim; maximum; may (v.2) "to take part in May Day festivities;" May; mayor; mega-; megalo-; mickle; Mister; mistral; mistress; much; omega.

It is the hypothetical source of/evidence for its existence is provided by: Armenian mets "great;" Sanskrit mahat- "great, mazah- "greatness;" Avestan mazant- "great;" Hittite mekkish "great, large;" Greek megas "great, large;" Latin magnus "great, large, much, abundant," major "greater," maximus "greatest;" Middle Irish mag, maignech "great, large;" Middle Welsh meith "long, great."
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*ghrei- 

ghrēi-, Proto-Indo-European root meaning "to rub." 

It forms all or part of: chrism; Christ; christen; Christian; Christmas; cream; grime; grisly; Kriss Kringle.

It is the hypothetical source of/evidence for its existence is provided by: Greek khriein "to anoint, besmear;" Lithuanian grieju, grieti "to skim the cream off;" Old English grima "mask, helmet, ghost," Middle Low German greme "dirt."

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*porko- 

Proto-Indo-European root meaning "young pig."

It forms all or part of: aardvark; farrow; porcelain; porcine; pork; porcupine; porpoise.

It is the hypothetical source of/evidence for its existence is provided by: Latin porcus "pig, tame swine," Umbrian purka; Old Church Slavonic prase "young pig;" Lithuanian paršas "pig;" Middle Dutch varken, German Ferkel, Old English fearh "pig, small pig."

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*pel- (2)

Proto-Indo-European root meaning "to fold."

It forms all or part of: aneuploidy; decuple; fold (v.); -fold; furbelow; haplo-; hundredfold; manifold; multiple; octuple; polyploidy; -plus; quadruple; quintuple; sextuple; triple.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit putah "fold, pocket;" Albanian pale "fold;" Middle Irish alt "a joint;" Lithuanian pelti "to plait;" Old English faldan "to fold, wrap up, furl."

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