Seed phrases explained

A seed phrase is a list of 12 to 24 ordinary words that can rebuild every key in your wallet. It is the master backup. It is also the thing thieves most want from you.

What it is

When you create a wallet, it generates a large random number. Writing that number down correctly would be error-prone, so the wallet shows it as words.

Most wallets follow a standard from 2013 called BIP39. It uses a fixed list of 2,048 English words. Each word stands for a small piece of the number.

12 words encode 128 bits of randomness.
24 words encode 256 bits.

Both are far beyond guessing. The last word also contains a checksum, which catches many copying mistakes. This is why you cannot make up your own phrase from words you like.

You may also see it called a recovery phrase, a backup phrase or a mnemonic.

One phrase, every key

From the seed, the wallet calculates all your private keys in a fixed order. The same seed always produces the same keys.

That gives the seed phrase two powers:

Recovery. Enter the phrase into a compatible wallet and your keys, addresses and balances return.
Total control. Anyone who enters the phrase gets the same thing.

There is no difference between your backup and your keys. Treat the words as the coins themselves.

The rules

Keep it offline. Write it on paper with a pen. Do not photograph it, type it into notes, email it or save it to cloud storage. Phones sync photos and files to online accounts, and those accounts get broken into.

Never enter it into a website. A real wallet asks for the phrase only when you choose to restore a wallet, inside the app or on the device.

Never give it to anyone. No support agent, exchange, developer or government needs it. Anyone who asks is stealing from you.

Keep the order. The words work only in the exact sequence shown. Number them as you write.

Check your handwriting. Each word on the list can be identified by its first four letters. Write clearly enough to read in ten years.

What can go wrong

Lost phrase, working device. You are one broken phone away from losing everything. Back it up now.
Lost phrase, lost device. The coins cannot be recovered by anyone.
Exposed phrase. Assume it is compromised. Move the coins to a wallet with a new phrase at once. Lesson 40 explains how.

A note on compatibility

Most wallets use the same standards, so a phrase from one usually works in another. It is not guaranteed. Different apps can derive keys in slightly different ways. If a restored wallet looks empty, check the wallet's documentation before you panic.

Lesson 30 covers how and where to store your phrase.