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I am a beginner of Bitcoins. I read here that the size of the blockchain is heavy HUNDREDS of gigabytes. I also read that one of the reasons bitcoin is safe is that EVERY user keeps a local copy of the blockchain on his machine and it gets updated whenever there is a transaction.

However, it is unthinkable to keep terabytes of data on the pc. How does it actually work?

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Not every user running a full node keeps a local copy of the entire blockchain. Most users probably run their node in pruned mode, which means they only need a few gigabytes to store the UTXO set (a set of transaction outputs that could be spent in future transactions).

The reason this is safe is that a pruned node needs to actually download and validate the entire blockchain when initially synchronizing. However since the UTXO set is enough to validate new transactions and blocks, the blockchain itself can then be discarded.

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