Oakland A's: Happy Roberto Clemente Day! - Athletics Nation
Uniform number (Major League Baseball) - Wikipedia
Roberto Clemente Award winners to wear cap patch
MLB's Puerto Rican-born players honor Roberto Clemente by wearing 21
Members of Red Sox wear No. 21 with pride on Roberto Clemente Day - The Boston Globe
Pittsburgh Pirates will all wear No. 21 to celebrate Roberto Clemente Day in 2020, per report
Curve Set to Retire No. 21 In Honor of Roberto Clemente
Report: Puerto Rican MLB Players Allowed to Wear No. 21 on Roberto Clemente Day, News, Scores, Highlights, Stats, and Rumors
Nats DH Cruz, other Clemente Award winners wear '21' on caps
MLB allows Puerto Rican players to wear No. 21 on Roberto Clemente Day - ESPN
Hall of Fame baseball player Roberto Clemente honored in new Paley Center for Media exhibit - CBS New York
Nats DH Cruz, other Clemente Award winners wear '21' on caps - The San Diego Union-Tribune
Pittsburgh Pirates, New York Mets players, coaches to all wear No. 21 on Roberto Clemente Day - ESPN