PDF417
PDF417 is a wide, flat code made of stacked bars, the one on the back of a driving licence or on an airline boarding pass.
What the word means
It is not a grid of squares but a stack of short striped lines. Each line is read like an ordinary striped code, and each carries its own row number, so a reader can put them back in order.
It holds several hundred characters where a striped code holds a number: a whole identity, a whole delivery line, a paragraph. It can also be read by a scanner swept across it, which the square codes cannot.
What it does on a real document
On a boarding pass it carries the passenger, the flight, the seat and the sequence number, in the exact order the airline industry fixed, so that any gate in the world reads the same pass.
It is wide and short, which suits the strip at the foot of a document. The engine draws it at the width it is given and stacks as many rows as the text needs.
Where it comes from, and why
It was invented in 1991 in the United States, at Symbol Technologies. Its name says what it was for: the three letters stand for portable data file, and the point was to carry the information itself rather than a number pointing at a database.
The number describes its own building block: each unit is four bars and four spaces, seventeen units wide. It is an international standard under the number ISO/IEC 15438, and administrations were the first to adopt it.