It's an odd number, befitting a brawny linebacker on the football field more than a graceful athlete in girls' lacrosse.
Marie McCool doesn't look like a No. 51.
Unless you know the history of that number for the Moorestown girls' lacrosse team - how it has been a tradition for that jersey to be passed down from one top player to another and how this No. 51 and that No. 51 have led the Quakers to state title after state title.
But here's the thing: McCool doesn't look like the other No. 51s, either.
"I've never had a player as dynamic as Marie," Moorestown coach Deanna Knobloch said Tuesday, on the brink of her 22d season in charge of perhaps the nation's premier program. "She makes this game look so easy."
There has been an assembly line of great players at Moorestown. The Quakers are a dynasty, pure and simple, with 51 consecutive victories and a 324-14 record and 13 state titles since 2000.
Knobloch and her husband and top assistant, K.C., won't come right out and say that McCool is the best player ever to wear one of those black uniforms.
There have been so many great ones, after all, and the special ones all had a unique quality. And besides, McCool still has to lead the Quakers to another state title and another Tournament of Champions crown.
So let's just say this: McCool probably plays the game with more functional speed, grace, fluidity, and maneuverability in tight spaces than anybody in Moorestown history.
And considering that she's a dynamite two-way player who actually prefers defense to offense, let's also say this: If McCool leads Moorestown to another undefeated season and becomes the program's first three-time all-American before departing for the University of North Carolina, well, the next No. 51 will have an awfully big jersey to fill.
"I'm trying not to think about that," McCool said of the responsibility of bequeathing the No. 51 to an underclassman after this season. "It's so much pressure. I know I don't want to hurt anybody's feelings."
McCool was given the jersey after the 2012 season by Stephanie Toy, now a sophomore at Notre Dame. All McCool did in her first season as the new No. 51 was score 86 goals with 26 assists as a junior and lead Moorestown to an undefeated record and another Tournament of Champions title.
"I was really surprised when Steph gave it to me," McCool said. "It puts the pressure on me, but that's a good thing for me. It makes me want to work harder. It makes me want to be a good leader."
McCool is such a natural that she was good at lacrosse before she even played lacrosse. She was more of a soccer player from kindergarten to third grade, and she used to tell her family she was going to play that sport for Notre Dame.
But she received a lacrosse stick as a Christmas present and went outside to toss the ball around with her father.
"I was like, 'I love this game before I even know how to play it,' " McCool said.
Knobloch said McCool was a "special player" as a freshman who has gotten better every year. McCool doesn't remember ninth grade that way.
"I was so scared," McCool said. "I remember when she pulled me up [to the varsity]. I was so nervous. It seems like yesterday, and now I can't believe it's my senior year."
Knobloch calls McCool "a gazelle" who is all but "unstoppable" on the field.
The coach says it's McCool's graceful athleticism combined with her versatility as a midfielder and her responsible two-way play that takes her game to another level.
Higher than all the other No. 51s?
It might be too soon to say, for sure. But there probably has never been a player who made such a square number look so sleek.
See and hear from Moorestown's Marie McCool.