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Q. What is the plural of a last name ending in a silent x? I just read an article using “the Robidouxes” and wondered if it should be “the Robidouxs” or “the Robidoux.” Thank you for your response.
A. See CMOS 7.11: “Names ending in an unpronounced s or x are best left in singular form.” Examples include “the seventeen Louis of France” and “The class included three Margaux.” So we would recommend the spelling “Robidoux” for both the singular and the plural. If for any reason you were to depart from Chicago and add an s for the plural, then either an es (as in the article you read) or an s—applied consistently—could work for names ending in x but not s (ss may not read as plural). If the final x or s is pronounced, an es would be required (e.g., “two Felixes”).