How does a QR code work?

Every UPI payment in India starts with a square of black and white dots. That square is a QR code — ‘quick response’ — invented in Japan in 1994 to track car parts.

A barcode stores data in one direction, as lines. A QR code stores it in two directions, as a grid of tiny squares, so it holds far more: a web address, a phone number, or a bank ID for payment.

The three big squares in the corners tell the camera where the code is and which way is up, so it can be scanned from any angle. The smaller patterns help the phone correct for a bent or dirty label.

QR codes also carry spare copies of their data. You can cover up to 30% of a code with a logo or a scratch, and it still scans. That is why shop codes still work with a thumb over the corner.