Almost always one of three things: it was printed too small, the two colours are too close in lightness for a camera to tell apart, or something was placed inside the quiet zone — the empty margin the code needs on all four sides.

All three look fine on a screen, which is why they are usually discovered after the flyers come back from the printer.

It depends on how much you put in it: more content means more squares, and smaller squares. That is why we work out the minimum for your code and print it under the preview instead of asking you to pick a size.

The number we give you is rounded up to the next half centimetre, and it is tested — we render at exactly that size to be sure it still reads.

Yes, from six gradients that have been checked. Each one is measured at its worst point rather than on average, because a gradient that scans at one end and fails at the other is still a gradient that fails.

The colours vary in hue rather than lightness for the same reason: a scanner reads light against dark, so a code that goes from light yellow to slightly lighter yellow has nothing to read.

• SVG if a designer is going to place it. It stays sharp at any size.

• PDF if it is going to a print shop. Same sharpness, and it carries a real physical size in millimetres.

• PNG for a slide, a document or a social post — anywhere that cannot take a vector file.

No. Not the text, not the URL, not a hash of either — a hash of a WiFi password is one lookup table away from the password.

When you download, we record the shape of the code and nothing else: the format, the size, how many bytes the content was, and a one-word category. The full details are on the privacy page.

No. The code you download contains your link directly, so it does not pass through us and there is nothing to expire, throttle or take down. Nobody has to keep paying for a printed flyer to keep working.