What is a QR code?

A QR code is a two-dimensional barcode that stores data as a grid of black and white squares. A camera reads the pattern and recovers the original text — usually a web address. It was invented in 1994 by Denso Wave to track car parts, and the format is free for anyone to use.

QR stands for "Quick Response"

The name is literal. The format was designed so a scanner could read it fast and from any angle — unlike a supermarket barcode, which must be presented roughly the right way up.

What is inside one

A QR code holds text. That is all. What makes it useful is that scanners recognise certain prefixes and act on them:

The text starts with	The phone offers to
`https://`	open a web page
`mailto:`	compose an email
`tel:`	dial a number
`WIFI:`	join a network
`BEGIN:VCARD`	save a contact
`geo:`	open a map
anything else	just display it

There is no lookup, no database and no server involved in a static code. The text is physically in the pattern.

The parts you can see

Three large squares in the corners — finder patterns. The scanner uses these to locate the code and work out its orientation. The missing fourth corner is how it knows which way up the code is.
Small squares scattered through larger codes — alignment patterns, which correct for the code being tilted or curved.
Dotted lines between the corners — timing patterns, a ruler that tells the decoder the grid spacing.
The blank margin — the quiet zone, which must be at least four modules wide or the code often will not be found at all.
Everything else is data and error correction.

How much it holds

At most 7,089 digits, 4,296 uppercase alphanumeric characters, or 2,953 bytes of arbitrary text. Those are ceilings, not targets: a code holding 2,953 bytes is 177 squares across and needs to be printed about 9 cm wide to be readable.

Who owns it

Denso Wave invented QR codes and holds patents, but chose not to enforce them against users of the standardised format. That decision is why QR codes became universal while comparable proprietary 2D codes did not. "QR Code" is a registered trademark; the format itself is described in the open standard ISO/IEC 18004 and is free to use.

FAQ

What does QR stand for?

Quick Response. The format was designed to be decoded fast and from any orientation, unlike the one-dimensional barcodes it was intended to replace.

Is a QR code free to use?

Yes. Denso Wave holds patents but has declared it will not enforce them against users of the standardised format. Generating and reading QR codes costs nothing and requires no licence.

What can a QR code contain?

Any text, up to 2,953 bytes. Scanners recognise prefixes such as https:, tel:, mailto: and WIFI: and offer the matching action; text with no prefix is simply displayed.

Why do QR codes have three squares in the corners?

They are finder patterns. Three points define an orientation, so the scanner can work out where the code is and which way up it is. A fourth would be redundant, and adding one breaks detection.