Useless fact
Why do bottle caps have twenty-one ridges?
The standard metal bottle cap on a glass bottle has exactly twenty-one ridges around its edge.
The crown cork, commonly known as a bottle cap, was invented by William Painter in 1892 as the first highly effective disposable sealer for carbonated beverages. Painter’s original design featured twenty-four ridges, or teeth, and a paper-backed cork liner to prevent the beverage from touching the metal. However, as bottling lines became increasingly automated in the early twentieth century, the cap was redesigned. An even number of teeth meant they sat directly opposite each other, which caused the caps to tilt and jam in the high-speed capping machinery. By reducing the number to an odd twenty-one teeth, manufacturers found the cap would align perfectly during sealing without tilting. Additionally, twenty-one ridges provided the optimal balance: enough grip to withstand the internal pressure of carbonated drinks, but not so tight that consumers could not easily pry them off. Today, this specification is standardized internationally under guidelines like DIN 6099, ensuring that glass bottles worldwide can be sealed reliably using the same machinery.
Why this is true — The design of the modern crown cap is standardized internationally with precisely twenty-one ridges to ensure a tight, even seal and prevent automated machinery jams.
Sources
- William Painter: Crown Bottle Cap National Inventors Hall of Fame
- US Patent US468258A: Bottle-sealing device United States Patent and Trademark Office