Bip39 Release Date Some words are shorter than 4 letters, including the word “act”. Other words are for a longer time than four letters, for example “abstract”. A single set of mnemonic can be employed to develop many addresses. This process of creating account was introduced in BIP32 & BIP39 https://thebookmarknight.com/story18525519/bip39-light-the-smart-trick-of-bip39-phrase-that-nobody-is-discussing