What's in a hand? Baseball is full of both right-handed and left-handed players, and the best to ever play the game are a mix of both.

There have been iconic lefty hitters from Babe Ruth to Ken Griffey Jr., and iconic righties from Willie Mays to Derek Jeter. The same goes for pitchers. There are all-time great lefties from Sandy Koufax to Randy Johnson, and all-time great righties from Bob Gibson to Pedro Martinez.

So what difference does it make? Well, it turns out being a lefty matters a lot, whether you're a pitcher, hitter or fielder. Here's why.

On the mound

A star left-handed pitcher can be a prize commodity. Why? For one, scarcity. There are just fewer lefties than than there are righties.

Think about it. Only about 10 percent of the general population is left-handed. That righty majority doesn't magically flip when it comes to baseball players. Take recent Major League Baseball history -- since 2010, nearly three quarters of all pitchers in MLB have been righties. Barely a quarter have been lefties. Teams will jump at the chance to get their hands on a great lefty pitcher.

But there's still the question of why a pitcher's handedness actually matters. The answer is what, in baseball, are called "platoon splits."

"Platoon splits" refers to a fundamental fact about baseball: Righty hitters do better against lefty pitchers, and lefty hitters do better against righty pitchers. It's a lot easier to see the ball coming from a pitcher of the opposite hand -- especially breaking pitches, which are easier to hit if they're coming toward you rather than moving away from you.

Need proof? Just watch this Chris Sale slider to Joe Mauer, one of the best left-handed hitters of his generation. It starts behind his back. It breaks all the way over the plate. He has no chance.

If you "split" a hitter's stats into how he does against righties vs. lefties, chances are, he'll have better numbers against pitchers who throw from the opposite side as he bats. Some teams will "platoon" two players at one position -- one right-handed hitter, the other a lefty -- to deploy depending on which kind of pitcher they're facing.

Now here's the thing that really makes lefty pitchers so valuable: The lefty platoon advantage is bigger than the righty platoon advantage. Lefty hitters do worse against lefty pitchers than righty hitters do against righty pitchers.

It makes sense. Over the course of a lefty hitter's baseball life -- from Little League to high school to college to the Minor Leagues to MLB -- he'll face many more righty pitchers than he will lefties. Less experience seeing lefty pitching likely makes it more difficult to hit when he does face it. Meanwhile, a righty hitter will see tons of righty pitching. He has the advantage of familiarity that comes from all those repetitions.

There are also relatively more left-handed hitters than there are left-handed pitchers. Righty hitters are still the majority, but it's not as big as it is for pitchers. So a lefty pitcher gets an outsized opportunity to take advantage of his platoon … uh, advantage.

In the Major Leagues in recent seasons, the proportion of righty hitters to lefty hitters is more like two-thirds to one-third, rather than three-fourths to one-fourth like for pitchers. And lefty hitters have taken over 40 percent of all at-bats, compared to the little over 25 percent pitched by lefties.

A lefty pitcher will find more lefty hitters to overmatch than a righty hitter will find lefty pitchers to attack.

At the plate

There are two clear benefits to hitting left-handed.

The first one goes back to the scarcity of lefty pitchers. A lefty hitter will get to bat in a lot more favorable matchups against righties than he'll have to bat in unfavorable ones against lefties.

The other: The left-handed batter's box is just physically closer to first base, and a lefty's swing takes his momentum in the direction he needs to run.

In fact, some parents will coach their kids to hit left-handed, or to become a switch-hitter, even if they're a natural righty. A young player might even teach himself to do it.

And when lefty swings look as sweet as they do, who could blame them?

In the field

Look around a Major League baseball field. You don't see any left-handed catchers. You don't see any left-handed shortstops or third basemen or second basemen. But you see righty throwers at every position.

The reason why is built into the physical design of the sport -- the direction players move around the diamond and the direction of most throws players have to make.

Let's start with the infielders. Most infield throws are to first base, to try to get the batter out. So a second baseman, third baseman and shortstop are all throwing to their left most of the time. That's the natural direction for a righty to step and throw. A lefty thrower would have to pivot awkwardly or throw with an unnatural motion. That costs valuable time, and a split-second can be the difference between safe and out.

There's a similar issue for a catcher. In particular, if a runner is stealing third, he's moving from the catcher's right to his left -- a natural throw for a righty, not a lefty. A stolen base can be decided by tenths of a second. There's no time to lose. There's an additional issue, too: Because most batters are right-handed, they'd be standing in front of a lefty catcher's throwing arm, making his throws more difficult.

The last lefty catcher played in 1989, when Benny Distefano filled in three times for the Pirates. And he's one of only five lefty catchers to play in a Major League game in the last century.

Lefty infield fill-ins are also rare. The Cubs' Anthony Rizzo, normally a first baseman, has played second and third during games the last few seasons -- but before him, the last lefty third baseman was Mario Valdez in 1997, and the last lefty second baseman was Don Mattingly in 1983. Lefty shortstops are the rarest. Shortstop is the most important infield position. The last lefty to play there was Mark Ryal in 1987.

Maybe one day, with defensive alignments becoming way more flexible than in the past, teams will break with tradition and lefties will play the positions that are all but off-limits today. But right now, for fielders, it's just like it is for pitchers and hitters: Being lefty or righty matters.