This site is named after this one small, widely misunderstood feature. The "25th word" is a passphrase you can add on top of a 24-word seed phrase. It offers real protection and a real risk, and it is worth understanding both. This is the starting article in our BIP39 passphrase guide.
What the 25th word is
A normal seed phrase turns directly into your wallet. The BIP39 standard allows one optional extra: a passphrase, which is mixed in when your wallet turns the words into a seed.
"25th word" is just a nickname, and it is a misleading one:
- It need not be a single word. It can be a whole phrase.
- It does not come from the BIP39 word list.
- Your wallet does not generate it. You choose it.
- With a 12-word phrase, it would really be a 13th word.
If you are new to the idea, our everyday guide to passwords and passphrases explains the general concept without any crypto.
How it works
Your wallet feeds your words and your passphrase through a one-way function. Change the passphrase by a single character and you get a totally different seed, and so a totally different wallet.
Under the standard, every passphrase is valid. There is no list of right answers and no stored check. Your wallet cannot tell you it is wrong. It simply opens whichever wallet that combination produces.
This gives two important results:
- Good: a thief with your written words sees only the standard wallet, which is empty if you keep your funds behind the passphrase.
- Dangerous: one wrong letter opens an empty wallet with no warning, which can look exactly as if you have been robbed.
What it protects against
| Threat | Does a passphrase help? |
|---|---|
| Someone finds your written seed phrase | Yes, this is its main purpose |
| A burglar takes your hardware wallet | Yes, as an added layer alongside the PIN |
| Someone pressures you to hand over your words | Partly, you can reveal a small decoy wallet |
| You type your phrase and passphrase into a fake site | No, both are stolen together |
| You forget the passphrase | No, this is a new risk it creates |
| Your family needs access after you die | No, it makes this harder unless you plan for it |
The biggest risk: forgetting it
Everything you have read on this site about there being no reset button applies twice over. If you forget your passphrase, even with all 24 words in hand, the funds behind it are out of reach. There is no support desk and no hint. Case, spaces and punctuation all count, so Blue and blue open different wallets. Read what is recoverable if you forget it.
Choosing a strong passphrase
The passphrase needs to hold up if a thief has your words. An attacker with your seed phrase can try passphrases on their own computers without any lockouts. The standard uses only 2,048 rounds of its key-stretching function, which slows that down only slightly.
So treat it as a master password, and read how to choose a strong passphrase. Use six or more truly random words, chosen the way we describe in how to create a strong password. Your pet's name, or any short word, offers little protection.
Should you use one?
Our full guide to that question goes through the pros and cons.
A passphrase suits you if all of these are true:
- You already have a tested, well-stored seed phrase backup.
- You hold enough that a found sheet of paper would be a serious loss.
- You can keep a separate, safe backup of the passphrase.
- You have told your family how to deal with it if you are gone.
If any of those are missing, fix them first. For most people, a good written backup does more than a passphrase ever will.
How to set one up safely
The exact steps vary by wallet, so follow your maker's official guide. The general approach is the same:
- Enter the passphrase on your hardware wallet's own screen if it allows it.
- Note the first receiving address that wallet shows.
- Before sending real funds, close the wallet, re-enter the passphrase and check you reach the same address.
- Send a small amount, then test it again from your backup.
Keep your words and passphrase off every website, form and chat. Nobody legitimate will ever ask for them, and that includes wallet support staff.