Nats DH Cruz, other Clemente Award winners wear '21' on caps
Mets, Yankees and more pay tribute on 9/11 20th anniversary
MLB Let Players Pick a Crazy Nickname to Wear. Their Choices Were Dull. - WSJ
Why are MLB players wearing 21 on their sleeve? - Quora
After Jackie Robinson, MLB integration was halting - The Washington Post
Houston Astros: 8 players don No. 21 on Roberto Clemente Day
Milwaukee Brewers: Christian Yelich Nominated for 2023 Roberto Clemente Award
MLB Clemente Award winners wear his 21 on caps for careers
Pirates, MLB honour 'the Great One' Roberto Clemente
Opening Day in MLB: New No. 21 patches, NL DHs and Guardians
Roberto Clemente: A Tribute to the Great One