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Seed phrase drill
A backup you have never checked is a guess. Practise the three checks that catch most mistakes, with practice words only.
Write down these 12 words
On paper, in order, with their numbers.
One word on this copy was written down wrongly. Which?
Compare it with your notes from check 1. This is what a careless copy looks like.
This check uses the practice words of check 1.
The last word carries a check
Each word stands for 11 bits. Twelve words make 132 bits: 128 bits of randomness and 4 bits of checksum calculated from them. If one word is mistyped, the checksum usually no longer fits, and a wallet refuses the phrase.
The check catches most single-word typos, not every one, and it cannot catch a valid word in the wrong position. That is why check 1 matters.
Create practice words to see their bits here.