The sobriquet Catholic King
is a title awarded by the Pope
as head of the Roman Catholic Church
to a monarch who in the eyes of the papacy embodies Catholic principles in his or her personal live and state policies. The title remains attached to monarchs descended from whoever received the original sobriquet, unless withdrawn by a pope. The sobriquet can be awarded to either a king or a queen. One of the rights of being a Catholic King
is that a queen may wear white, rather than the normal black, when meeting a pope.
Two surviving European monarchies have the sobriquet Catholic King: those of Belgium and Spain (since Los Reyes Católicos).
See also Fidei defensor and