A key written on paper survives a dead drive, a lost phone, a locked account and a company going out of business. What it does not survive is being typed back in wrong: one character, and there is nothing to tell you which one. This lays a secret out in an alphabet with no letters that look alike, puts a check on every line and one across the whole sheet, and reads it back telling you exactly which line is wrong.
One line per line on the sheet. Row numbers, spaces and case do not matter, and the letters that are easy to confuse are accepted either way round.
The alphabet has no letters that look like each other. There is no I, no L, no O and no U on the sheet. Zero and one can only be digits, so a one written in a hurry cannot be read as a letter, and a handwritten correction cannot turn into a different character. This is the same alphabet used for anything meant to be read aloud or copied by hand.
Every line carries its own check. Three characters at the end of each line are worked out from the line and its number. Change any single character, or swap two round, and the line stops matching. That turns "it does not work and I do not know why" into "line four is wrong", which is the difference between a backup and a rectangle of paper. Three rather than two because two would let roughly one mistake in a thousand through, and a wrong key you believe in is worse than no key.
The whole sheet has a check too. Lines can each be right while the sheet is still wrong: a line left out, a line typed twice, two lines in the wrong order. The value across the bottom catches all three, and is checked after the lines are put back together.
Paper is not a safe, and this does not pretend otherwise. What is printed is the secret itself, readable by anyone holding the page. That is the right trade for a recovery phrase in a locked box at home and the wrong one for anything left on a desk. Encrypt it first with LOCKBOX and print that, or split it across several sheets with SHARD, if it needs to survive being seen.
It happens in this tab. The secret is laid out in the browser and printed from the browser. Nothing is uploaded, which for the one piece of text that must never be uploaded is not a small point. Use a printer connected directly rather than one that queues jobs through somebody else's service.
Anything up to a few kilobytes: a passphrase, a recovery phrase, a private key, a small key file. Large files are the wrong thing for paper, and the page says so rather than producing forty pages nobody will ever type.
Reading it back tells you which line and stops there rather than handing you a key that is silently wrong. That is the whole reason for the check characters: a wrong answer that looks right is worse than no answer.
No, and deliberately not. Correcting errors automatically means guessing, and a guess that happens to satisfy a check is a corrupted secret you now believe in. It tells you where to look and you fix it from the paper in your hand.
No. It is the secret, in plain characters, made easy to read by design. Treat it the way you would treat the key itself: a locked box, a safe, a bank. If it has to survive being seen, encrypt it before printing or split it across several sheets.
Yes, and the words themselves come back exactly as they went in. Many people prefer to write the words down directly, which is fine; the reason to use a sheet like this is the check on every line, which a handwritten list does not have.