Double Spend

Double spending is using the same digital token twice before the network treats the first spend as final. Digital data copies easily. Physical cash does not. Before blockchain, a bank or PayPal kept a central ledger. That trusted party stopped double spends because it owned the books.

Blockchain stops double spending without that party through distributed consensus. You broadcast a transaction. Nodes relay it. Miners or validators put it in a block. Once confirmed, honest nodes reject a second spend of the same coins because those outputs are already spent. Reversing a confirmed transaction means rewriting history, which means majority consensus power, a 51% attack.

Extra confirmations make reversal much harder. Merchants wait based on size. A $5 sale might take one confirmation. A $50,000 transfer waits for six or more. Double-spend attacks still happen on weak chains or during partitions. Major chains have not seen successful double spends at meaningful scale.

Double spending is the core problem of digital currency: spending the same digital token twice before the system detects the duplication. Unlike physical cash, digital information can be copied infinitely. Before blockchain, the only solution was a trusted intermediary, a bank, PayPal, or payment processor, that maintained a central ledger tracking who owns what.

This trusted party could prevent double-spends because they controlled the single source of truth. The Bitcoin paper's core problem is double-spending. The chain plus proof of work is the proposed fix.