# Interleaving

> Interleaving is the reordering of QR code data so that consecutive codewords come from different error-correction blocks. It spreads localised physical damage across multiple blocks, so no single block exceeds its correction capacity.

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

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## Why it matters in the real world

Damage to a printed code is almost never randomly distributed. It is a scratch, a fold, a
coffee ring, a torn corner — contiguous. Without interleaving, a contiguous blot would
destroy one block entirely and be unrecoverable, even though the code as a whole had plenty
of spare correction capacity.

With interleaving, that same blot takes a few codewords from each of several blocks, and
every block stays within its correction limit.

## The practical consequence

A thin scratch across an entire code is often recoverable. A solid blot covering the same
total area is more dangerous, because it concentrates damage per block after
de-interleaving. And damage over the finder patterns is worst of all — those are not data
and cannot be corrected at all.