Yankees to retire Paul O'Neill's No. 21: See the complete list of retired numbers
NY Yankees royalty honored as Paul O'Neill's No. 21 is retired
New York Yankees history: Paul O'Neill's number 21 must be retired
Mailbag: Retired Numbers, Hall of Fame, Lyerly - River Avenue Blues
The All-Unretired Yankees, Bronx Pinstripes
Yankees to dedicate Mariano Rivera's Monument Park plaque – The Denver Post
New York Yankees: 5 numbers that need to be un-retired
Yankees want to stop issuing uniform numbers to managers, coaches because they're running out
List of New York Yankees captains - Wikipedia
Yankees retired numbers: A look at all of the jersey numbers players cannot wear in the Bronx
Here are the Yankees' 21 retired numbers Who belongs and who doesn't? Andy Pettitte, Jorge Posada, Bernie Williams, more