What it looks like and where it goes
An alignment pattern is a 5 × 5 dark ring, a 3 × 3 light ring and a single dark module in the center. Version 1 has none, version 2 has one near the bottom-right corner, and the count grows as the code gets larger.
The patterns sit at fixed positions listed in the standard for each version. They are placed on a regular grid, skipping any spot that would overlap a finder pattern.
- Version 1: none
- Versions 2–6: 1
- Versions 7–13: 6
- Versions 14–20: 13
- Versions 21–27: 22
- Versions 28–34: 33
- Versions 35–40: 46
Why they are needed
Finder patterns alone are enough to locate a small code, but a large code photographed at an angle or printed on a curved surface is distorted. Alignment patterns give the reader extra reference points so it can map each module to the right place.
The one near the bottom-right is especially useful because that corner has no finder pattern. Damage to an alignment pattern is often recoverable, but it makes decoding harder than damage to ordinary data modules.