What it means that a phrase «checks itself»

The account number example

A bank account number carries a couple of digits that aren’t information: they are a check calculated from the rest. If you get one figure wrong, the calculation doesn’t add up and the system spots it instantly, without asking anyone and without a connection.

Your recovery phrase works the same way. Part of the last word isn’t a free word: it is a check on everything before it. That is why, out of every imaginable combination, only a few are valid phrases.

How much it discards

Translated to your case: if you are missing one word of twelve and don’t even know the position, there are 2048 possible words in each of the twelve slots, but only one in sixteen combinations passes the filter. That leaves about fifteen hundred candidate phrases. A computer goes through them in seconds.

What the filter doesn’t do

• It doesn’t tell you which valid phrase is yours. They are all legitimate and each opens a different wallet. That requires checking which one holds your money.
• It doesn’t save you if you don’t know the order. Discarding fifteen in sixteen is useless against a twenty-four-digit number. See I have all 24 words but not the order.
• It doesn’t protect against a forgotten extra password. That isn’t part of the phrase and isn’t checked at all. See the «25th word».
• It doesn’t recover a completely lost phrase. A filter discarding 94 % of attempts is irrelevant against a forty-digit number.

That is exactly the boundary of what can be done: the filter turns «part is missing» into a small problem, and does nothing at all for «nothing is left».

Keep reading

• I’m missing one word of my 12-word phrase: can it be recovered?
• What a recovery phrase actually is (and what it isn’t)
• 12 or 24 words? Why it matters when you need to recover
• Can a completely lost phrase be recovered? The honest answer