Baseball, by the Rules
A top-down diagram of a baseball field and the surrounding area. Everything here is roughly to scale. Roughly.
The Field
The field. The ballpark. The diamond. The playing surface of a baseball game can go by a few different names, but no matter what you call it, when it comes to MLB ballparks, each field will have some elements that are exactly the same as other baseball fields, and other elements that are unique. First, let's talk about the field itself— specifically, the ground that makes up the playing surface. Most MLB parks use a combination of natural grass and dirt, though some parks swap that grass out for artificial turf1. As for what you find on the field, well, the diagram here gives you an idea of the full field, but as you might be able to tell, some parts of the field require a little bit of a zoom-in. So, let's do just that, and start with:
Home Plate
Home plate might be small (17 inches wide), especially in comparison to the rest of the field, but it's where everything starts in terms of how the field is laid out. Immediately next to home plate are the two batter's boxes, where the batters stand when they are up to bat. One thing to keep in mind: a right-handed batter will bat from the right-handed batter's box, but if you're looking at things from behind the plate (or over it as suggested by our diagram), that batter's box is on the left side. If you look at it from the picher's vantage point, however, a right-handed batter will be on the right side of the plate, and a lefty will bat from the left.
Radiating out from the batter's boxes are the first- and third-base foul lines. The area outside of these lines, and behind the plate itself, is considered foul territory, and we'll discuss why that's important in a little bit.
Behind the plate is the catcher's box, and if you're already guessing that the catcher goes there, you're correct.
If we zoom out from home plate, we come to...
The Infield
The infield consists of home plate, the pitching mound (where the pitcher stands when pitching), the bases, and the base paths. The playing surface here consists of dirt around home plate, throughout the base paths, and all over the pitching mound, with grass (or artificial turf) everywhere else2. The infield is where all offensive play happens; a batter starts at home plate, and then attempts to progress from base to base, in a counter-clockwise fashion, until they reach home again, scoring a run.
The specific dimensions for the infield are spelled out in the rules, where the infield is defined as a square, with a base (or home plate) at each corner, and sides of 90 feet.— but of course, this square is usually seen as being perched on its tip, thus it looks like a diamond, which is why one calls the field itself "the diamond." The pitcher’s mound is a circle 18 feet in diameter, with the pitching plate (more commonly called the pitching rubber) sitting exactly 60 feet, 6 inches from the back point of home plate. The pitcher’s mound, meanwhile, peaks at 10 inches above the surface of the field, with a gradual slope down on all sides. The bases are 18 inches square, and between 3-5 inches thick. The outer edge of the dirt, meanwhile, extends in an arc 95 feet from the center of the mound, with the interior boundary being up to the whims of the local groundskeepers 3.
Once you have gone past the infield dirt, you are in the realm of...
The Outfield
The outfield consists of the area between the edge of the infield dirt, and the outfield wall, with the two foul lines serving as an additional boundary. The entire outfield is covered in grass (or turf), except for a strip of dirt around the outfield wall. This is called the “warning track,” and serves to yes, warn outfielders that they are about to run into the aforementioned wall. The wall, meanwhile, is where you will see the greatest amount of variation between ballparks. While MLB rules specify certain minimums in terms of outfield depth4, the league frequently grants exceptions to these rules5. Some variation in dimension comes from how ballparks have been wedged into tracts of land that are not quite large enough, so the planners were forced to get creative with the space. More recent ballparks usually do not have any such limitations, yet the designers have chosen asymmetrical outfield walls for the “quirkiness” of it all. Similarly, there are no firm rules on how high an outfield wall can be, which leads to a lot of variation6. The outfield itself is divided into thirds (left field, center field, and right field), though there are no actual boundary lines as such.
Just as no two outfield walls are the same, the area behind those walls varies from park-to-park. Most MLB parks include two "bullpens" directly behind the outfield walls. These bullpens (one used by the home team and one used by visitors) are where relief pitchers sit through most of the game, then warm up before they come in to pitch. Aside from the bullpens, MLB parks feature outfield stands, various concourses, and other gathering areas. However, no park will have fans congregating in the area directly behind center field. Why? Because that area serves as the batter's eye— an area where the stadium will have a dark background of some sort. This makes it possible for batters to track pitches as they come out of pitchers' hands. Were it not for the batter's eye, hitting would be almost impossible at the MLB level.
Last but not least, we have...
Foul Ground
The area outside of the foul lines is known as foul ground or foul territory, as opposed to the field of play proper, which is called “fair” territory. The foul lines run from the batters’ boxes around home plate, past first and third base, to and up the outfield wall, before terminating in a pair of poles that are designed to help show if long fly balls are fair (which means they are home runs), or mere foul balls. The line itself, including the bases and the poles, are considered to be in fair territory. Defenders cannot be in foul ground when play starts (save for the catcher, who is required to start there), but can range into foul territory to make plays. Foul territory is bordered on the outside by the stands, and by the dugouts. The stands are where spectators sit and watch the game. Dugouts sit below ground level, and serve as a spot for each team to congregate, allowing players to rest when they’re not on the field. Both the stands and the dugout are considered out of play. There are no regulations as to how much or little foul territory a ballpark can have; generally speaking, newer ballparks tend to have less foul territory than older ones as architects attempt to get seats closer and closer to the action. The more foul territory you have, the more room for defenders to turn certain types of foul balls into outs.
Oh, and one more thing, here— if you've been paying attention to the various field diagrams, you might have spotted two circles lingering around in foul territory, down near home plate. Those are the on-deck circles, and they are where the on-deck hitter waits and prepares before coming up to bat. And who is the "on-deck hitter"? Well, it's the batter that happens to follow whomever is currently at-bat. Whomever that is will come to the on-deck circle, prepare their equipment, stretch a bit, and watch pitches come in, trying to get a sense of how those pitches move, and what sort of timing is going to be involved in hitting them. Meanwhile, the batter who is hitting after the on-deck hitter will be waiting, "in the hole," often loitering near the steps of the dugout. When the current plate appearance ends, the on-deck hitter strolls to the plate to start their own plate appearance, while the batter in the hole heads out to the on-deck circle, and the cycle continues.
The only specifications the on-deck cicle falls under is that each circle must be 6 feet in diameter. Their location doesn't matter, as long as they are safely out of the way, and at this point, most circles are, infact, rubber discs that players can move around at will, even if this might get them in trouble. In fact, most times, batters won't actually stand in the on-deck circle, so much as stand relatively near it.