# Finder patterns — how a scanner locates a QR code

> Finder patterns are the 7×7 concentric squares in three corners of a QR code: a 3×3 dark centre, a light ring, and a dark ring. Any straight line through the centre crosses dark and light in a 1:1:3:1:1 ratio, so a scanner can find the code at any rotation.

Source: https://useqr.app/docs/spec/finder-patterns · Last reviewed 2026-08-21 · UseQR is free forever, MIT licensed, no signup.

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## The structure

Each [finder pattern](/glossary/finder-pattern) (the standard calls them *position
detection patterns*) is 7 × 7 modules:

- a **3 × 3 dark centre**,
- a **1-module light ring** around it,
- a **1-module dark ring** around that.

Read across the middle and the widths are **1 : 1 : 3 : 1 : 1** — one dark, one light,
three dark, one light, one dark. A one-module light **separator** then isolates the whole
pattern from the data region, so the finder plus separator occupies an 8 × 8 corner zone.

At version 1, the three finders alone are 147 of 441 modules — exactly one third of the
symbol. Location comes before data.

## Why the ratio is rotation-invariant

The clever part is geometric: **any straight line through the centre of concentric squares
crosses the rings in the same 1:1:3:1:1 proportion** — horizontal, vertical, or at 40
degrees. A scanner does not need to find squares at all. It streams pixel rows looking for
runs of dark and light in that ratio, checks the candidate in the perpendicular direction,
and it has found a centre. That works whatever the rotation, which is why you can scan a
poster upside down.

The ratio is also deliberately rare in real imagery and in the code's own data — the
[mask penalty rules](/docs/spec/mask-selection-penalty-scores) charge a heavy 40-point
penalty for accidental 1:1:3:1:1 runs in the data region, precisely so false finders do not
appear.

## Why exactly three

Three points determine position, rotation and scale. With finders at top-left, top-right
and bottom-left, the *missing* fourth corner tells the decoder which way is up: the corner
without a finder is always bottom-right. A fourth finder would add no orientation
information and would cost another 64-module corner. (The fourth corner's job of handling
perspective distortion goes to the cheaper 5 × 5
[alignment patterns](/docs/spec/alignment-patterns) instead.)

## What finder patterns will not survive

Reed–Solomon error correction protects only the data region. **Cover one finder and the
code is gone** at any error-correction level, because the scanner never gets far enough to
use the redundancy. This is the hard rule behind
[logo placement](/docs/design/qr-code-with-logo-best-practices): centre is negotiable,
corners are not. It is also why a sticker, staple or fold across a corner is so much worse
than the same damage in the middle — see
[scanning a damaged code](/docs/scanning/how-to-scan-a-damaged-qr-code).

Styling the finders ("custom eyes") is a partial exception: decoders tolerate rounded or
reshaped eyes **as long as the concentric dark-light-dark structure and its proportions
survive** in a thresholded camera image. Hollow eyes, gradients that cross the rings, or
low-contrast eye colours break the ratio and the code with it. Always
[decode-verify](/glossary/decode-verify) a styled code before shipping — [try it](/validate).

## FAQ

### What are the three squares in the corners of a QR code for?
They are finder patterns that let a scanner locate the code and determine its orientation. Any line through their centre crosses dark and light modules in a 1:1:3:1:1 ratio, which software can detect at any rotation angle.

### Why do QR codes have three squares and not four?
Three points are enough to fix position, rotation and scale, and the empty fourth corner itself signals orientation. Perspective correction is handled by smaller 5×5 alignment patterns, which cost far fewer modules than a fourth finder would.

### Can a QR code work with a damaged finder pattern?
Usually not. Error correction only protects the data region, so a covered or heavily damaged finder prevents the scanner from locating the code at all. Minor edge damage sometimes survives because detection needs the ratio, not pixel perfection.

### Can you change the shape of the corner squares?
Within limits. Rounded or restyled eyes scan when the concentric dark-centre, light-ring, dark-ring structure and its 1:1:3:1:1 proportions are preserved after thresholding. Test the result with a real decoder before printing.

## Try it

- https://useqr.app/url
- https://useqr.app/scan
- https://useqr.app/validate