The largest, longest-lasting hack ever?

Hack, in the sense of "workaround", created by humans, elegant or not, and not restricted to computing.

My current contender is upper and lower case letters in English. It's like mixing two fonts in the same document. If scribes and early printers wanted ways to break up text, making different shapes for the same letters is an odd way to do it. Maybe space constraints were their real problem?

And what about double quotes? Someone came up with the idea of using a small mark before and after a word or phrase to set apart. Then someone else decided to just use the same marks twice. I'm surprised we haven't seen triple and quad quotes appear over the centuries.

What's your vote for the longest-living and largest hack in the history of humanity?