New York Yankees: The Numbers on the Wall
NY Yankee Paul O'Neill's No. 21 to be retired on Aug. 21
Where Derek Jeter ranks among Yankees' retired single-digit numbers
As Paul O'Neill's No. 21 Is Retired, One Last Kick of a Water Cooler - The New York Times
Why don't the Miami Marlins have any retired numbers?
New York Yankees: Retiring numbers, plaque ceremonies must end
Source: No decision yet about when Derek Jeter's No. 2 will be retired - Newsday
New York Yankees: 5 numbers that need to be un-retired
As Paul O'Neill's No. 21 Is Retired, One Last Kick of a Water Cooler - The New York Times
Yankees Retired Numbers - Who Are The 22 Yankees Who Wore Them