OK, I admit it, I'm a baseball rulebook lawyer. I just think that baseball rules are a fascinating topic; there are so many eccentricities and twists and turns. Not to mention that the book itself is bizarrely organized, and a lot of times you have to look in notes and comments sections to find out what the actual intent of a rule is. There's a lot of stuff that we, as baseball fans and maybe sometime players, have absorbed and unconsciously apply, which usually works, but then we get tripped up on unusual situations. I used to do some softball league umpiring, and I always had a rule book on me, because at least two or three times a season, it seemed like some "never saw that before" situation came up. I'm going to post a series of Rules Roundup articles between now and the end of spring training. Some of them will be ones I've posted before, but have been revised to note rule changes since they were written.
Let's look at the rules that cover the specific dimensions and features of the playing field. Rules 2.01-2.05 contains some words detailing the layout of the field, but most of the pertinent info is contained in three drawings in Appendix 1, which rule 2.01 references. The "root" of a ballfield layout is the "point" of home plate, the corner that points back towards the backstop. The two sides of the point are at 90 degree angles to each other, and at 45 degree angles to the front edge of the plate (the edge closest to the pitcher). All dimensions of a ball field are referenced to this point, if the field is laid out properly. We'll also talk about some of the playing rules that make reference to dimensions, markings or features of the field layout.
The Bases and Foul Lines
The foul lines are drawn such that their outside (foul side) edges come to a point at the point of home plate, or they would if the line was drawn all the way to the plate itself. (Modern field layouts usually do not chalk the portion of the base line that goes through the batter's box.) The angle between the foul lines is 90 degrees. The chalked lines themselves are in fair territory. The lines are chalked all the way from home plate to the outfield fence. (Regarding lines in general, artificial turf fields usually have all lines that are in grass areas painted on the turf, rather than chalked.)
First base and third base are placed with a side on the foul line, up against the outside edge of the line. Since the bases are square, this means that another edge will be facing home plate. The distance to the base is measured to the side opposite of that one; the "back" side of the base is exactly 90 feet from the point of home plate. (The 2023 change to the size of the bases means that the "front" edges are now 3" closer to home plate than they used to be.) The base lines going from first base to second base, and second base to third base, conceptually extend from these edges, such that they are at 90 degree angles from the two base lines that they intersect with. (In a modern field layout, the lines from first to second, and from second to third, are not actually chalked, but they are measured, to make sure second base is in the right place.) Second base is placed so that its center (not a corner) is over the point where these two lines meet, behind the pitcher's mound. It is oriented so that one side faces first base and one side faces third base; one of its corners points toward home plate. All bases except for home plate are 18" square (changed from the traditional 15" foinr 2023).
The "three-foot" line is the line in foul territory parallel to the first-base line, and it extends from halfway between home and first, to first base. If the batter is trying for first, and the defense is attempting a play on the batter, then the batter must run between the infield grass (2024 rules change; the boundary used to be the base line) and the three-foot line. If the batter runs outside of the three-foot line or on the infield grass, and by doing so the defense is prevented from making a play, then by definition it is interference and the batter should be called out. (There is a reference in the MLB press release to "18 to 24 inches" and allowing ballparks with turf fields some time to make modifications. This is confusing to me, because the width of the base path is supposed to be standardized, so I’m sure why the additional space is not precisely known or why any field would need modifications.)
2023 rules specify that an imaginary line that extends from the point of home plate through the center of second base divides the infield in half. At the time a pitch is delivered, there must be at least two infielders (not including the pitcher and catcher) on either side of this line.
Fun fact: All four bases are entirely within fair territory. If a ground ball touches first or third base, it is a fair ball, regardless of where it goes afterwards. This does not apply to home plate, but a batted ball is not foul just because it touches the plate. You often hear the phrase "fouled off the plate" used by announcers, but this is just in reference to the fact that a ball that is hit straight down such that it bounces off the plate will often have a spin on it that will cause it to bounce into foul territory. If a batted ball touches the plate and stays in fair territory, it is a fair ball.
The Infield and Outfield Grass
On a baseball field,the area of the infield inside of the baselines is mostly grass. (On a regulation softball field, the infield is all dirt; this is an easy way to tell a softball field apart from a baseball field from the air.) Along the first base and third base lines is a dirt area for the baserunning path, 6 feet wide and centered on the baseline. Along the two second base lines, the area starting 3 feet along the inside of the line is dirt, and the dirt continues out to the edge of the outfield grass, which is a 95-foot radius drawn from the center of the pitching rubber. 2023 rules specify that no more than three players may be positioned on the outfield grass at the time a pitch is delivered.
At first, second and third base, there is also a "notch" in the infield grass. Home plate and the batters' box area are contained within a 26-foot diameter dirt circle centered on the point of home plate. (During the heyday of artificial turf fields in the 1970s, it was common to see infields that were entirely turf except for the home plate area, the pitcher's mound, and a "sliding box" around each of the other bases. The rule book apparently no longer allows this, if it ever really did, but at one time it was almost universal on turf fields.)
The Batter's Boxes, The Pitcher's Mound, and Other Field Features
Each batter's box is 5 feet from front to back, 4 feet wide, and its inside edge is 6" from the closest edge of home plate. A line drawn left and right through the center of the plate exactly cuts the box in half. Note that a corner of the box is in fair territory. If a batter makes contact and then accidentally interferes with the ball in fair territory, while still within the box, it is a foul ball and the batter is not out. The catcher's box has its front edge at the point of home plate; it extends 8 feet back from here, and is 43" wide. Seeing as to how the catcher's box no longer has any bearing on the rules (now that it is no longer necessary to throw pitches to give the batter an intentional walk), some groundskeepers do not bother chalking it.
The pitcher's mound is a 18-foot dirt circle, situated so that it straddles a line drawn between the point of home plate and the center of second base. A 60x34" area on top is (in theory) level. The pitching rubber is placed within this area, and is situated such that the edge closest to home plate is 18" behind the center of the mound, which puts it 60-1/2 feet away from the point of home plate. When you look at a field, it may appear that the pitching rubber is positioned exactly between first base and third base, but it is not so; if you draw a line between the center of those two bases, the rubber is roughly 3 feet closer to home plate than the line is. The height of the mound, since 1969, is 6 inches. (Traditionally, it was higher. It was lowered after the 1968 season produced some of the lowest offense totals in modern-era history.) From the front edge of the level area, the slope is supposed to fall at a rate of 1 inch per foot, such that the outermost three feet or so of the mound is level with the infield.
The coaches' boxes are 20 feet long and must be 15 feet away from the base lines. (Note that base coaches are normally not confined to the boxes during a play, per rule 5.03(c). The coach may, for instance, run down the line in order to communicate with the baserunner. However, if a coach commits interference, the opposing team can request that coaches be confined to the boxes for the remainder of the game, and then the umpires will enforce it.) The on-deck circles are located by drawing a line left and right from the rearmost point of the home-plate circle, and the center of each on-deck circle is 37 feet from that point. (The on-deck circles used to be closer. They were moved to their current location after Steve Yeager, a catcher for the Dodgers, was hit in the throat by the jagged end of a broken bat while on deck in a game in 1976. Yeager survived, and became a World Series hero for the Dodgers in 1981.)
Foul territory, as we know, is not strictly specified by the rule book. Rule 2.01 suggests, but doesn't mandate, that in the infield, there should be at least 60' between the foul line and the backstop/fence/wall that separates the playing field from the spectator areas. Many ballparks now have netting down the lines to protect spectators in the first level from foul balls and bats that slip out of a batter’s hand The construction and dimensions of such nets is not specified in the rule book.
The Outfield and Miscellaneous Field-Related Rules
The rule book has very little to say about outfield dimensions. There is nothing in the book about the warning track, although other provisions of MLB rules usually require ballparks to have a track that is roughly 15 feet wide. In the early 20th century, before warning tracks were common, some ballparks had sloped areas leading up to the outfield wall. Houston’s Minute Maid Park tried to re-create this; it originally had a feature called "Tal’s Hill" in center field. Players hated it, and it was removed in 2016 and replaced with a conventional warning track.
Outfield depths, and the shape and height of the outfield wall, are left up to the individual ballpark. Rule 2.01 provides that any park built or remodeled after 1958 is required to have dimensions of at least 325' to the fence down the foul lines, and 400' to dead center; however, several contemporary ballparks, notably the new Yankee Stadium, do not comply with this rule. Rule 5.05(a)(5) states that in order for a ball hit over the outfield fence in fair territory to count as a home run, the distance from the plate to the outfield wall must be at least 250 feet; if it is less than that, the hit is a double only. There is currently no MLB stadium where this applies.
A ballpark's ground rules may cover additional situations or oddities that may be present. A few examples:
The Cubs' ground rules for Wrigley provide that a fly ball hit into the "basket" that overhangs the outfield wall is a home run. A bouncing ball into the basket is a double.
The Rays' ground rules for Tropicana Field cover situations where a batted ball hits one of the ballpark's notorious catwalks. Some are home runs and some are in play, depending on which catwalk it hits.
At Fenway Park, a fair ball that goes through an opening in the manual scoreboard, either on the fly or on the bounce, is a double.
When Citi Field first opened, the Mets had a ground rule for it that stated that a batted ball that hit the Pepsi sign overhanging the field was a home run. The sign has since been moved and the rule no longer applies.
The Minnesota Metrodome had a rule that a ball that went through a roof exhaust vent was a foul ball. They put that rule in after Dave Kingman hit a pop-up that went out through one of the vents, and nobody knew what to do.
At Truist Park, some of the dugout railings are painted silver. These are defined as being entirely within the dugout. A ball that hits one of them is out of play.
Foul poles are not specified in the rule book. They are left up to the individual ballpark, and each park's ground rules cover how the foul poles are to be interpreted.
A ballpark may have the distances from home plate to the outfield wall painted on the wall in a maximum of five places: at the two foul lines, left center, right center, and dead center. Teams are prohibited from using additional markings in the outfield for the purpose of providing position or distance measurements for the offense or defense to take advantage of. (Of course, advertising on the wall can be used for the purpose; the rule book is silent on that.)
Roof Opening and Closing
A few special rules apply to retractable-roof stadiums. The decision to start a game with the roof open or closed is the home team's choice. If a game starts with the roof open, or if it is opened during the game, and then it becomes necessary to close it due to weather conditions, the home team must first notify the chief umpire, who notifies the visiting team. If the visitors agree, the roof is closed; if the visitors disagree, the chief umpire makes the final decision. After the start of a game, once the roof has been closed, it may not be re-opened.
If the game begins with the roof closed, then the home team may request of the chief umpire that the roof be opened if weather permits. If the visiting team agrees, then the roof will be opened; if not, the chief umpire makes the decision. Opening of a closed roof is not permitted after the start of the seventh inning. The roof opening must take place in between innings. At Chase Field, due to the amount of time required, it is done in two parts: the first part is done at the end of an inning, and the rest is done at the end of the next inning.