“Rational” is an adjective meaning “reasonable” or “logical”: “Ivan made a rational decision to sell his old car when he moved to New York.” “Rational” rhymes with “national.”

“Rationale” is a noun which most often means ”underlying reason”: “His rationale for this decision was that it would cost more to pay for parking than the car was worth.” “Rationale” rhymes with “passion pal.”

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