How a seed phrase works
The words encode a random number. Your wallet turns that number into a master key and derives every private key and address from it, following the BIP-32 standard.[2]
Each word stands for an 11-bit number, so the list has exactly 2,048 words. The first four letters of each word are enough to identify it, so a backup that records four letters per word is still complete.[1]
The phrase is turned into a 512-bit seed with PBKDF2, 2,048 rounds of HMAC-SHA512, using the word "mnemonic" plus your optional passphrase as the salt. That seed becomes the BIP-32 master key.[1]
Types of seed phrases
Most wallets use 12 or 24 words from the BIP-39 word list. Some let you add an optional passphrase, sometimes called a “25th word”, which creates a different set of wallets from the same words.[1]
BIP-39 allows five lengths. A 12-word phrase holds 128 bits of randomness plus a 4-bit checksum; 15, 18 and 21 words hold 160, 192 and 224 bits; a 24-word phrase holds 256 bits plus an 8-bit checksum. Ledger devices use 24 words.[1]
Every passphrase produces a valid wallet, so a typo in the passphrase opens an empty wallet instead of showing an error. BIP-39 calls this plausible deniability; the cost is that the wallet cannot tell you the passphrase was wrong.[1]
How a seed phrase is used
You write it down when you create a wallet and use it only to restore access on a new device. It is the core of self-custody: no company holds a copy.
Risks and scams
Fake support agents, phishing sites and fake airdrops all try to get your words. ethereum.org says no legitimate service, support agent or website will ever ask for your recovery phrase.[8]
Seed phrases and US taxes
A seed phrase has no tax consequences by itself. The IRS treats digital assets as property, so keep records of transfers and sales; restoring a wallet does not change what you own.[4]
Seed phrase vs private key
A private key controls one address; a seed phrase can regenerate all of them. Losing one key loses one address; losing the phrase loses the whole wallet if you also lose the device.
Example: one phrase, many addresses
Illustrative example: after you enter a phrase, an Ethereum wallet usually derives the first address at m/44'/60'/0'/0/0 and the next at m/44'/60'/0'/0/1. The 60' is Ether's coin type in SLIP-0044.[5]
When you restore, a wallet may scan for used accounts. BIP-44 sets a gap limit of 20: discovery stops after 20 unused addresses in a row. If you only used address number 25, you may need to add accounts by hand before its balance shows up.[6]
The checksum catches most typos. The last word partly encodes a SHA-256 checksum of the entropy, so a wrong word usually makes the wallet reject the phrase as invalid instead of opening a different wallet.[1]
How to store it
Keep the phrase offline, on paper or on a metal plate that survives fire and water. Ledger advises never typing a hardware wallet's phrase into a software wallet, because that puts the keys on an internet-connected device.[7]
Do not take a screenshot or a photo. ethereum.org warns that screenshots can sync to cloud storage, where they are exposed if that account is breached.[8]
Common mistakes
Typing the phrase into a site that offers to "sync", "validate" or "claim an airdrop". ethereum.org lists fake airdrops and support impersonators on Discord as common ways wallets get drained.[8]
Keeping the only copy next to the device. If both are lost in the same fire, the funds are gone. If only a hardware wallet breaks, entering the phrase into a new compatible device restores every account.[7]
The Bottom Line
A seed phrase is the one backup that matters in a self-custody wallet. Write it down offline, keep it private and never share it.
Educational content, not financial advice.