Rational is an adjective: based on reason, sensible — a rational decision. Rationale is a noun: the set of reasons behind something — the rationale for the decision. The final E makes it a noun, and shifts the stress to the end: ra-shuh-NAL.
The core difference
An adjective and a noun, one letter apart.
- rational (adjective) — reasonable, logical: a rational explanation.
- rationale (noun) — the reasoning behind something: explain the rationale.
The pronunciation tell
Rational stresses the first syllable. Rationale stresses the last and rhymes roughly with morale — another noun English marked with a final E for exactly the same reason.
The mathematical sense
A rational number is one expressible as a fraction of two integers. It uses the "ratio" sense of the root rather than the "reasonable" one — Latin ratio covered both reckoning and reason, which is why the two senses share a word.
Frequently asked questions
Which one is the noun?
Rationale, with the final E. Rational is the adjective.
How is rationale pronounced?
Stressed on the last syllable, roughly ra-shuh-NAL, rhyming with morale.
Why is a fraction called a rational number?
From the ratio sense of the Latin root, not the reasonable one — ratio covered both reckoning and reason.