On this website, we can see that the the private key 1 is written in this format.
I would like to know what is this format.
Bitcoin Stack Exchange is a question and answer site for Bitcoin crypto-currency enthusiasts. It only takes a minute to sign up.Sign up to join this community
This is the Base58 Wallet Import format.
It is composed of the alphanumeric characters excluding
0OIl (zero, capital O, capital I, and lowercase L). The format includes an error checking code which makes it highly unlikely to mistype a key.
The format is also in use for addresses. Private keys start with a five. Pay-to-pubkey-hash addresses start with a one, pay-to-script-hash addresses start with a three.