If you can demonstrate to me how something works, then it must be demonstrable. You might think that word should be "demonstratable," but you'd be wrong — that word doesn't exist.

Who can say why some words change a little when we change their part of speech. They just do. The verb demonstrate means to show or instruct. The adjective shortens the root to become demonstrable. The steps for buckling a seatbelt must be easily demonstrable, or people will go flying all over the cabin. Demonstrable is often used to refer to the truth of something: if something isn't demonstrable, then we can't really say that it exists, can we?

Definitions of demonstrable

adj capable of being demonstrated or proved

“a demonstrable lack of concern for the general welfare”
easily perceived by the senses or grasped by the mind

adj necessarily or demonstrably true

demonstrable truths”
incontestable, incontestible
incapable of being contested or disputed

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