⭐ Merited by Findingnemo (1)
Moreover, 12 words is easier if someone wants to memorize them (never forget to make physical backups, memorizing the mnemonic is just a plus).
The passphrase is very important, because even if the generated seed has low entropy, I can increase the security of the mnemonic by extending it using passphrase.
For passphrase generation, I recommend using 6 random words or more. Random words of 6 words have, for example, the same entropy as a password like "#\KL`@cZ3W*-", I usually think that 8 or more is great.
The passphrase is very important, because even if the generated seed has low entropy, I can increase the security of the mnemonic by extending it using passphrase.
For passphrase generation, I recommend using 6 random words or more. Random words of 6 words have, for example, the same entropy as a password like "#\KL`@cZ3W*-", I usually think that 8 or more is great.
Do not trust your brain to remember the 12 words phrases and not writing them down somewhere. There are many chances that one may forget all or any word of the phrases and you will be deprived of your money stored in that wallet. DO NOT TAKE THAT RISK.
Secondly, when you extend the phrases, it is better to add three to four easy words rather than complex words with special characters etc. Easy English words will do the purpose. Keep the things as simple as possible.
1 - Write down the mnemonic phrase on a piece of paper (or steel plates for greater durability).
2 - Write down the passphrase somewhere else offline, in a separate offline location away from the mnemonic phrase, because if an attacker finds your mnemonic phrase, but your funds are protected by a hidden wallet generated by the passphrase, he will not be able to steal from you.
3 - Make copies of the backup (if in digital format, make sure to encrypt it).
If seeds are created with low entropy then bruteforcing would require less time and the passphrase may not be just enough to stop the attack unless we are talking about a long sentence used as the 13th word. But I don't recommend either of these practices, it's better use the wallet itself to create the valid phrase whether it's Bip39 or others and the extra word can be something that you can remember easily, so it's better to be 8 characters than 8 words. 
Extending the seed, extra word = BIP39 passphrase, they're the same thing. I recommend reading this informative guide [https://www.useapassphrase.com/] to better understand the use of passphrases.