# Reed–Solomon coding

> Reed–Solomon is the error-correcting code that lets a damaged QR code still decode. It adds redundant codewords so that a decoder can reconstruct the original data even when part of the pattern is unreadable.

Source: https://useqr.app/glossary/reed-solomon · Last reviewed 2026-08-21 · UseQR is free forever, MIT licensed, no signup.

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## How it applies here

QR codes use Reed–Solomon over GF(256). The data is split into blocks, each gaining a
number of error-correction codewords determined by the version and level. Because the
scheme can correct **erasures** (known-missing symbols) at twice the rate of **errors**
(wrong symbols), a decoder that can tell "this region is obscured" recovers more than one
that just reads noise.

That is the technical reason a logo works: the decoder sees a clean, obviously-covered
region rather than scattered bit errors.

## Interleaving

Codewords from different blocks are interleaved before being placed in the matrix, so a
single physical scratch spreads its damage across blocks rather than destroying one block
entirely. This is why a thin scratch across a whole code is often survivable while a solid
blot of the same total area may not be.