List of Major League Baseball retired numbers - Wikipedia
Nats DH Cruz, other Clemente Award winners wear '21' on caps
MLB's Puerto Rican-born players honor Roberto Clemente by wearing 21
Yankees rally in the ninth to slip past the Pittsburgh Pirates 7-5 on Roberto Clemente Day
Roberto Clemente day: Puerto Rican MLB players, Pirates wear No. 21
ESPN: MLB to allow Pirates, Puerto Rican players, award nominees to wear No. 21 on Roberto Clemente Day