Chiasmus is a rhetorical figure in which two or more clauses are balanced against each other by the reversal of their structures. The elements cross like the letter X.

“Ask not what your country can do for you—ask what you can do for your country” is a famous modern instance. The second half inverts the grammatical order of the first, creating a memorable symmetry. Classical and biblical writers used chiasmus extensively; it appears in poetry, oratory, and prose whenever a writer wants to give a thought the feeling of completeness or inevitability.

Chiasmus differs from simple antithesis by the specific device of inversion. The crossing of terms can highlight contrast, reinforce a paradox, or simply please the ear with its patterned return. When the inversion feels forced, the figure becomes merely clever; when it grows naturally from the thought, it seems both elegant and true.

A sentence that turns back on itself by reversing its own structure is employing chiasmus.