On this day in 2013, Marek Palatinus (Slush) and Pavol Rusnak proposed BIP 39 — Mnemonic Code for Generating Deterministic Keys — the standard behind the 12 or 24-word seed phrases used by nearly every Bitcoin wallet today.

Combined with BIP 32 (HD wallets), it meant that your entire Bitcoin wallet could be backed up by simply writing down a set of words on a piece of paper.

It remains one of Bitcoin's most important usability innovations.