The two generations of the QR symbology: the original Model 1 (versions 1–14, never ISO-standardized) and Model 2 (versions 1–40 with alignment patterns), which is the QR code everyone uses today.
QR codes exist in two models, and virtually every QR code you have ever scanned is Model 2. Model 1 was Denso Wave's original 1994 design; Model 2 is the enhanced 1997 revision that became the ISO standard. The distinction matters today mainly for compatibility archaeology — old readers, legacy specifications, and the occasional exam question — but understanding what changed explains several features of the modern symbol.
What Model 1 looked like
Model 1 defined versions 1 through 14, topping out at a 73×73 module symbol. It had the three finder patterns and timing patterns familiar from Model 2, but no alignment patterns — the grid had no local distortion correction, which limited reliable reading to flat surfaces photographed fairly straight-on. Its format information layout and version encoding also differed, and its maximum data capacity was a fraction of Model 2's. Denso Wave published Model 1 as a company specification; it was never submitted to ISO.
What Model 2 added
Model 2 extended the version range to 40 (177×177 modules), introduced the 5×5 alignment patterns that make curved-surface and angled reading practical, reworked the format and version information for better error protection, and refined the masking and error-correction partitioning. Critically, Model 2 was standardized as ISO/IEC 18004 (first edition 2000), which is what turned QR from a Japanese automotive-industry tool into a global symbology with interoperable readers. Every "QR code" generated by modern software, printed on packaging worldwide, and decoded by phone cameras is Model 2.
Telling them apart
In the wild, Model 1 symbols are essentially extinct — you will encounter them only in legacy industrial systems installed before the 2000s. If you do find one, the giveaways are the absence of alignment patterns on a symbol larger than Version 1 and a subtly different finder-pattern separator geometry. Modern decoders are generally Model 2-only; many will fail on Model 1 symbols, which is one more reason legacy systems get upgraded rather than maintained.
Why the history matters for practitioners
Three practical lessons survive from the Model 1 era. First, the version-14 ceiling is why some very old documentation mentions 73×73 as the maximum QR size — if you inherit such a document, update it. Second, alignment patterns exist because Model 1's real-world failure on anything but flat, straight-on surfaces taught the industry that local distortion correction is non-negotiable; this is the same reason you should never cover alignment patterns with logos today. Third, the Model 2 format-information design (with its own error correction) is why modern codes survive damage to the metadata areas that would have killed a Model 1 symbol. When someone asks whether "QR codes" support a feature, the answer is almost always about Model 2 — and Model 2 is the only model worth generating.
Diagram description: Side-by-side Model 1 (version 14, no alignment patterns) and Model 2 (version 14, alignment grid present) symbols with the structural differences annotated.