Backing up Bitcoin using only private keys? So I'm really new to Bitcoin and cryptocurrencies, and I don't truly understand realtion between Bitcoin private and public key, and address!

I understand that for every private key, there is a public key, and that public key can be made and is made from a private key. So for every transaction you would have one private key and corresponding public key.

What I can't understand is how to backup Bitcoin wallet using private key! 
Everyone says you just need to backup your private keys, but do I need to backup every private key, or just the first one, or i just need one private key to access my wallet?

Lets say i would like to backup my Bitcoin wallet by writing private keys on a piece of paper and storing that somewhere safe:


Do I need to save only first ever created private key to have access to all bitcoins I own, even i added them through multiple transactions?
Do I need to save every single private key generated, from every transaction to have access to all Bitcoins on blockcain?
If I save only half of private keys, from half of my transactions, does it mean i can only access Bitcoins from those transactions i own private keys?


I'm really confused about this one, and cant find answer anywhere! I'm not interested in backing up wallet, i need to store my private keys on a paper! 

Sorry if question seems silly, I'm an old guy trying to learn all this new wonders! Thank you!
It depends on what kind of wallet you have.

Traditionally each private key was generated randomly by your wallet software. In this case every private key needs to be backed up.

Recently deterministic wallets have become popular. Deterministic wallets randomly generate a seed, and then use that seed to deterministically derive the private keys in a cryptographically secure way.

To answer your questions specifically:


It depends on whether you use a deterministic wallet. Traditional method: If you make multiple transactions, new keys are periodically generated and these need to be backed up too. Deterministic wallet: backing up only the seed is sufficient.
Also depends on wallet. Traditional: back up every key. Deterministic: just backup the seed.
Yes. Private key ownership = ownership of the bitcoin held at the addresses derived from those keys. If you don't have the key, you can't spend the bitcoin.


In Bitcoin Core, both kinds of wallet can be sufficiently backed up by backing up your wallet.dat file. There is an HD (Hierarchical Deterministic) symbol in the bottom right that is greyed out and crossed out if you do not have a deterministic wallet.

Since you want to backup on paper, maybe Bitcoin Core is not the best solution for you. You could try using a wallet that implements BIP 39 (there is a list of them on the deterministic wallet article), which derives the seed from a human-readable list of words. In this case, writing down the seed words is a sufficient backup.