Bip 39 github https://giphy.com/channel/ebliciwsah Although it employs BIP32 approach you may still import BIP39 seeds into Electrum wallet. Also other than BIp32 There exists BIP44 Model which some wallet use for the generation of seeds. For words that have only 3 letters, there isn't any 4th letter. As an example https://atavi.com/share/xbwf7vz16jgof