English word sequence demonstrating lexical ambiguity

That that is is that that is not is not is that it it is is an English word sequence demonstrating syntactic ambiguity. It is used as an example illustrating the importance of proper punctuation.[1]

The sequence can be understood as any of four grammatically correct sequences, each with at least four discrete sentences, by adding punctuation:

That that is, is. That that is not, is not. Is that it? It is. That that is, is that that is. Not is not. Is that it? It is. That that is, is that that is not. Is not "is that" it? It is. That that is, is that that is not, "is not." Is that it? It is.

The first, second, and fourth relate a simple philosophical proverb in the style of Parmenides that all that is, is, and that anything that does not exist does not. The phrase was noted in Brewer's Dictionary of Phrase and Fable.[2]

This phrase appeared in the 1968 American movie Charly, written to demonstrate punctuation to the main character Charly's teacher, in a scene to demonstrate that the surgical operation to make the character smarter had succeeded.[3]

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