Why QR Codes Have Three Squares in the Corners
The three corner squares are there so a camera can tell which way is up.
Look at any QR code and three of the four corners carry a big square inside a square inside a square. The fourth corner is empty. That asymmetry is the point.
When a camera finds a QR code, it doesn't know which way the code is oriented. Upside down, sideways, mirrored — the image could be any of those. The three corner squares, called finder patterns, solve it. A scanner hunts for them first, then uses which corner is missing to figure out rotation.
The squares aren't just there to be squares. The black-white ratio along a scanline crossing one is always 1:1:3:1:1. That exact sequence almost never occurs naturally in a photograph or on a printed page, so the decoder can lock onto it quickly even in a busy image.
Masahiro Hara designed the format at Denso Wave in 1994. He needed a code that Toyota assembly-line cameras could read from any angle while parts slid past at speed. The three-square layout came from staring at the black-and-white tiles on office buildings outside his window, looking for a pattern a computer could find in a glance. Denso released the spec without charging royalties, which is why QR ended up everywhere.
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