Glossary
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.
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.
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