What is a passphrase? A extra word on your Mnemonic that generates an totally different wallet.

!!Warning!!

Using a mnemonic and a passphrase to generate private keys is an advanced setup that doesn't came without its challenges.
You should test it over and over it again until feel comfortable and feel secure about this setup, before use it for large amount of funds.

I will advice you to test this setup on sparrow wallet. Even as a hotwallet. With a just a few satoshis.
Use it every day to train yourself. Practice basic stuff like export/import (backup creation and restore) your wallet, never forgetting what mnemonic standard the wallet is using (BIP39 or not) and the derivation path.
This info should also be stored always with your mnemonic and passphrase: the mnemonic standard, the derivation path, the software/hardware used, the date, an identifier for the wallet (e.g: Son 18 years account and etc...), an identifier for the wallet for multisig (if it is supported).

Another alternative is to use Electrum. You can even use Tails OS in a DVD and test this setup, since Tails OS already bring Electrum installed. Use it in the same way I describe for Sparrow.

With time as your confidence and experience increases, you could try a mobile wallet that supports passphrase.

On creating a passphrase don't use spaces. The best option is follow the idea in this comic https://xkcd.com/936/ and more info about it on this gist https://gist.github.com/maximusmaximalistus/c68fe3f2d3f751142cc8b50431a07049
IMHO one should only use letters, upper case and lower case, and numbers.

Always check authenticity and integrity of your software. The authenticity can be done with PGP. The integrity can be done with a checksum calculator that uses an hashing algorithm (md5, sha256, sha512, ...)

#Blockstream article: What is a passphrase
https://help.blockstream.com/hc/en-us/articles/5131416184601-What-is-a-passphrase-

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