On April 15, 1977 — 20 years to the day before baseball recognized and embraced the date’s significance as Jackie Robinson’s debut anniversary — Hank Aaron broke important new ground himself.Just more than three years after he broke Babe Ruth’s home-run record, the now-retired Aaron had his No. 44 retired by the last team for which he played, the Milwaukee Brewers. That made him the first player ever to have two teams honor him in that way, after the Braves, for whom he had played for 21 years in Milwaukee and Atlanta. MORE: Watch ‘ChangeUp,’ a new MLB live whiparound show on DAZNNumber retirements weren’t exactly routine back then, either, so while it was far from a stop-the-presses moment for sports, it did set him apart in a unique way. Since then, number retirements have grown and the rules (such as they were) have become more flexible. The entire concept of retiring numbers has been challenged, for the most basic of reasons: Teams are running out of numbers.With all that in mind, here are 10 notable MLB number retirements, starting with the beginning of the tradition itself.— The first number ever retired by a baseball team was No. 4, by the Yankees, in honor, of course, of Lou Gehrig in 1939 (he made a pretty famous speech about how lucky he was that day). Fast-forward 80 years: Now, every single-digit jersey is retired by the Yankees, including No. 8 twice (for catchers Bill Dickey and Yogi Berra).— No. 42, as most fans surely know by now, is retired throughout baseball in honor of Robinson. The last active player to wear it is going into the Hall of Fame this summer: Yankees closer Mariano Rivera, who last wore it in the 2013 season.— Since Aaron was honored, two players have had numbers retired by three teams: Frank Robinson (Reds, Orioles, Indians) and Nolan Ryan (Angels, Astros, Rangers).— Three players have at least two teams retiring two different numbers in their honor: Ryan (Angels 30, Astros and Rangers 34), Carlton Fisk (Red Sox 27, White Sox 72) and Reggie Jackson (A’s 9, Yankees 44).— Two managers have numbers retired by two teams: Casey Stengel (Yankees, Mets) and Sparky Anderson (Reds, Tigers).— Besides the Yankees, the Cubs have one number retired in honor of two players: No. 31, for pitchers Ferguson Jenkins and Greg Maddux.— One franchise has never retired a number in honor of one of its players: the Florida/Miami Marlins, born in 1993. Their only currently retired number is 42 (for Robinson). It once had honored team co-owner Carl Barger, who died before the team’s first-ever game, by retiring No. 5 (for his favorite player, Joe DiMaggio), but returned it to circulation in 2012. There are still tentative plans to retire No. 16 for pitcher Jose Fernandez, killed in a boating accident in 2016. — The lowest uniform number to be retired is 1, by eight teams. The highest for a player is 72, for Fisk by the White Sox. The highest period is 85, for late Cardinals owner Augie Busch, who turned 85 the year of his ceremony in 1984.— Four Montreal Expos’ numbers are being honored by the Montreal Canadiens. The Expos moved to Washington in 2005, and in 2015 the Canadiens raised a banner in Bell Centre with the numbers of the Expos’ honorees: Gary Carter, Rusty Staub, Andre Dawson and Tim Raines.— The Cardinals retired Bruce Sutter’s No. 42 nine years after the sport-wide retirement of the number for Jackie Robinson.