The Private and the Public Key pair can be compared with a password and bank account. As term "Public" suggests, Public key or Bitcoin address is a unique personal address, that is shared in the blockchain, and everyone can see it, while Private key works as a password, that you must keep just for yourself.

Keys are inseparable, based on mathematical functions (like prime number exponentiations, elliptic curves, hashing functions and many other) and connected into a complex algorithm which is irreversible, in other words, works one way:

                                                                                        Private Key → Public Key / Bitcoin address

The Public Key contains a string of letters and numbers, usually generated from the Private Key, that identifies the sender or the recipient of the funds. Common Public Key (Bitcoin address) starts with number 1 and requires one Private Key to unlock the funds. The newer version of the Public Key, called multi-signature Public key, starts with number 3 and to reach the coins, more than one Private Key must be provided.

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FORMAT EXAMPLE:

Public key (Bitcoin address) 1DSsgJdB2AnWaFNgSbv4MZC2m71116JafG

Multi-signature public key 3J65t1WtEk25CNmQviecrnyiWadfRhWNLy

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Public keys are generated from 26 to 34 characters, and in order to prevent visual ambiguity and fraud, uppercase letter "O" and a number "0", uppercase letter "I", lowercase letter "l" are excluded from the set.