Why the end zone is exactly ten yards deep

Before 1912 there was no end zone. The goal line and the end line were the same line, and you scored by crossing it.

The forward pass created a question

Once passing was legal, rulemakers had to answer something nobody had needed to think about: how far behind the goal line could a pass legally be caught? Their answer was to draw an end line ten yards back and name the space between the two the end zone.

Then the arithmetic broke

The playing field was already 110 yards long. Adding a ten-yard end zone at each end would have made the whole thing 130 yards, which would not fit inside some existing stadiums.

So they deleted the 55-yard line.

On 3 February 1912 the NCAA's rules committee cut the field of play from 110 yards to 100 by removing the 55-yard line — the old midfield — and appended the ten-yard end zones, keeping the 160-foot width.

What it means

The most iconic number in the sport, the 100-yard field, is not a design choice. It is what was left after the end zones were bolted on and the stadiums refused to get any bigger. And the fifty-yard line is the middle of the field only because a different line used to be.