$BTC
Bitcoins can currently be subdivided by seven decimal places: a thousandth of a bitcoin is known as a milli and a hundred millionth of a bitcoin is known as a satoshi.
In truth there is no such thing as a bitcoin or a wallet, just agreement among the network about ownership of a coin. A private key is used to prove ownership of funds to the network when making a transaction. A person could simply memorise their private key and need nothing else to retrieve or spend their virtual cash, a concept which is known as a “brain wallet”.
