First 4 Letters of a BIP39 Mnemonic Seed Phrase

Why do I only need the first 4 letters for each word in a mnemonic seed phrase?

Your 12-24 recovery seed phrase most likely comes from the Bitcoin Improvement Proposal: 39 (BIP 39). BIP 39 is the use of a mnemonic phrase -- a group of easy to remember words -- to serve as your back up recovery in the event your wallet becomes compromised.

These words are pulled from a specific list of 2048 words.  In this list, the first 4 letters are unique to each word. That means if you have the first 4 letters, you know the rest of the word. Some wallets will even fill in the rest of the word once the first 4 letters are entered.

Below is an exercise using Google Sheets to count the number of unique words in the BIP39 English wordlist using only the first four letters of each word. We ensure that it also equals to 2048. This is also known as a mnemonic frequency analysis.

Take a look or try it for yourself.