If the contestant agrees or disagrees correctly, their symbol (X or O) is placed in the square; otherwise, their opponent's symbol is placed there. To win the game, however, a player cannot take advantage of their opponent's mistake; they must respond correctly on their own turn.
Since the center square tends to get picked most often, generally the most famous or amusing celebrity is placed there. The late Paul Lynde was the center square for the program's original run between 1968 and 1981; other center squares of later versions have included Joan Rivers, Whoopi Goldberg and Ellen DeGeneres.
Tom Bergeron has hosted Hollywood Squares since its revival in 1998. Peter Marshall (1966-81) and John Davidson (1986-89) have hosted previous versions.
A UK version of the show, renamed Celebrity Squares, appeared on ITV in the 1970s and 80s, hosted by Bob Monkhouse.