Etymology
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*kad- 
Proto-Indo-European root meaning "to fall."

It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."
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*tag- 
Proto-Indo-European root meaning "to touch, handle," with figurative extensions ("border on; taste, partake of; strike, hit; affect, impress; trick, cheat; mention, speak of").

It forms all or part of: attain; contact; contaminate; entire; intact; integer; integrate; integrity; noli me tangere; tact; tactics; tactile; tangent; tangible; task; taste; tax; taxis.

It is the hypothetical source of/evidence for its existence is provided by: Latin tangere "to touch," taxare "to touch, assess," tactus "touch," integer "intact, whole, complete, perfect; honest;" Greek tassein "to arrange," tetagon "having seized;" Old English þaccian "stroke, strike gently."
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