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3 years ago i bought a little bit of Bitcoin. Today i found the private key of my wallet. But I cant access it, I need these 12 words (backup) I dont have. Why cant I access my Bitcoin Wallet with Private Key? (Like Myetherwallet, here you can access wallet with private key)

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You just need to find some software that can import your private key.

These days, the standard for backing up wallets is defined by BIP32/39/44/49/84, and it comes in the form of a 12 or 24 word mnemonic seed phrase. This seed phrase is used to derive all keypairs and addresses in the wallet, so you just need to store it safely in order to recover your wallet later.

So most wallets accept a seed phrase, and inputting raw private keys has become less common (though it is definitely still possible). You just need some software that can accept a private key.

Note that in some cases, you may have to create a new wallet, and then import the private key to it— there may not be an option to create a new wallet by importing the private key alone.

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With bitcoin core it's pretty easy to import a single private key. You just open the console (menu "Window"->"Console") and type in:

importprivkey "<the private key in WIF>" "somelabel"

WIF here means Wallet Import Format and is a standard way of encoding a private key. It should start with either K, L or 5. Be aware that this will start a full chain rescan to look for UTXOs associated with that private key which can take up to several hours to complete depending on the speed of your disk.

If you know exactly the block where the first UTXO was in (from a block explorer perhaps), you can also do:

importprivkey "<the private key in WIF>" "somelabel"
rescanblockchain <startblockheight>

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