After
Louis XVI, king of
France, was
deposed and the French
Republic declared on September 21st 1792, he obviously ceased to be king. However, his line had long since shed things as
profane as
family names, relying on titles to uniquely identify people. This posed a bit of a problem for the revolutionaries when they tried him for
treason during the following winter: it would have looked rather silly to hold a trial against just "Louis" (a very common name), but they didn't want to use any filthy
aristocratic titles. The name of Louis' House,
Bourbon, was probably considered inappropriate for the same reason.
Thus, the Committee of Public Safety went back to the last dynasty of
French kings to have simple family names, the Capets, and used that name, turning Louis XVI into "Citoyen Capet" (citoyen, "citizen" was the politically correct salutation of the time, superseding the aristocratic Monsieur), who was sentenced to death on January 18th 1793 and guillotined in the Place de la Révolution in Paris on January 21st 1793.