Bip39 backup https://www.plurk.com/p/3hf2syltjx Although it takes advantage of BIP32 method you could even now import BIP39 seeds into Electrum wallet. Also besides BIp32 There's BIP44 Model which some wallet use for that technology of seeds. For words that only have three letters, there isn't a 4th letter. One example is https://geilebookmarks.com/story20516031/bip39-bip32-bip39-compatible-wallet-options-https-x-com-tundra838554