Home Plate
Seventeen inches of rubber that defines the strike zone, the foul lines and the entire geometry of the field.
What it actually is. Home plate is a five sided slab of whitened rubber, seventeen inches across the front edge, set flush with the ground. It is the point everything else is measured from: the foul lines run from its back corners, the bases are laid out from it, and the strike zone is defined as the space above it.
What somebody who knows the game gets out of it. Seventeen inches is the number that decides what pitching is. A ball is just under three inches across, so a pitch that catches the outside edge is using about ten percent of the plate's width, and the difference between a strike and a ball on the corner is roughly the width of a seam. That is why command is measured in inches rather than in general accuracy, why the automated ball strike system is defined as a percentage of the plate rather than a picture, and why hitters who can cover both edges are so rare that clubs build entire scouting reports around where a man cannot reach.
The numbers worth knowing. Seventeen inches wide at the front, sides 8.5 inches, back point formed by two 12 inch edges. Add a baseball's width to each side and a pitcher has about 22.9 inches of legal target.
What people get wrong about it. The plate is fair territory and it is part of the field, so a bunt that stops on it is fair. Also, the strike zone's width is the plate plus any part of the ball, so a pitch that clips the very edge with the very edge of the ball is a strike by rule even though it looks off the plate on television, where the broadcast box is usually drawn to the plate itself rather than to the legal zone.
At Little League and travel ball. The plate is seventeen inches at every level, from tee ball to the majors, which is one of the few dimensions that never changes. Everything else on the field scales with age and the plate does not, which is why a wider zone is called at younger ages: relative to a nine year old's reach, seventeen inches is a much larger target.
Batter's Eye: look up anything in baseball