What is the difference between TD1, TD2, and TD3 MRZ formats?

TD1, TD2, and TD3 are the three ICAO-defined layouts for the Machine Readable Zone. The difference comes down to line count, character count per line, and which document types typically use each one.

TD1 is a three-line format, 30 characters per line, most commonly found on national ID cards. Because ID cards are physically smaller than passports and often carry data on both sides, TD1 was designed to fit a compact block of text on the back of a card while still holding a full set of identity fields plus check digits.

TD2 is a two-line format with 36 characters per line, sitting in between the other two in terms of capacity. It shows up less often than TD1 or TD3 but appears on some ID cards, visas, and other travel documents where a full passport-style MRZ would be too wide for the available space, but a three-line ID-card format isn't the convention either.

TD3 is the two-line, 44-characters-per-line format used on passports. It's the one most people picture when they hear "MRZ," the long strip across the bottom of the passport's data page.

Passports need to encode slightly more information reliably given how central they are to international travel and border control, so TD3 has the most characters per line of the three formats.

For a scanning system, correctly identifying which format is present is the very first step, and it happens automatically before any field extraction begins. The software effectively needs to recognize "this is a three-line block, so it's likely TD1" or "this is a two-line 44-character block, so it's TD3" before it can know where the date of birth or document number sits within the string.

Getting this classification wrong means every field afterward gets misread, since the position of each field depends entirely on which format is in play.

This is one of the less visible but genuinely important parts of a mature document reader. ScanDoc's engine handles all three formats along with their many real-world variants, since documents from different countries don't always follow the ICAO templates with perfect consistency, and a reliable reader has to accommodate that variation rather than assuming every document is a textbook example.