Glossary
Finder pattern
The three large concentric squares in the corners of a QR code are finder patterns. A scanner locates them first to work out where the code is, how big it is and which way up it is — which is why the missing fourth corner is deliberate.
The 1:1:3:1:1 ratio
Each finder pattern is a 7 × 7 module block: a black 7 × 7 square containing a white 5 × 5 square containing a black 3 × 3 square. Scanned along any line through the centre, the black-white ratio is 1:1:3:1:1 — a signature that is rare in natural images and survives rotation, which is what makes QR codes findable at any angle.
Why only three corners
Three points define an orientation; a fourth would be redundant. The empty bottom-right corner is how a scanner knows which way up the code is. This is also why you cannot "balance" a design by adding a fourth square — it breaks detection.
Styling them
Finder patterns tolerate rounding and colour changes provided the 1:1:3:1:1 ratio survives at the sampling resolution and contrast stays high. Filling the centre square with a logo does not survive. UseQR verifies styled output by decoding it before download, which is the only way to know a styled finder still reads.
Related
- Alignment pattern — Alignment patterns are the small 5 × 5 concentric squares scattered through larger QR codes. They let a scanner correct for perspective distortion when…
- Timing pattern — The timing patterns are the alternating black-and-white lines running between the finder patterns, one horizontal and one vertical. They tell the decoder…
- Module — A module is one square cell of a QR code — the smallest unit of the pattern. Codes are measured in modules, not pixels or millimetres, because the module…