On July 21, in picturesque Cooperstown, New York, four former Major League Baseball players, Roy Halladay, Edgar Martinez, Mike Mussina, and Mariano Rivera, will get inducted into the Baseball Hall of Fame. Rivera is the only one of the four, and the only one of all 323 inductees of all time, to be unanimously voted in.

Rivera’s induction is no surprise, but the unanimity of the writers is startling considering that of the 226 former players in the Hall of Fame, only six of them were relief pitchers. What is it about Rivera?