I was reading this GitHub book on Bitcoin

They state that "True or False: Only UTXOs can be used as inputs to transactions."

Is the answer, True because you can only spend what you have.

Or False, you can out a spent output in the transaction as input, but the network or the miner will reject it.

  • If everyone rejects your transaction, can you call that spending?
    – alcio
    Mar 19 '18 at 17:56
  • @alcio no, you can still use it for an input though right? There's nothing stopping you Mar 19 '18 at 18:00
  • In a technical sense, yes, you are not limited to the UTXO set for inputs, but in a practical sense, you are if you want your transaction to confirm.
    – alcio
    Mar 19 '18 at 18:31

This question makes no sense.

Of course you can make an invalid transaction that doesn't spend an unspent output. If you're fine with calling invalid things transactions, I could just take a piece of random data and call it a transaction.

A valid transaction can only spend unspent outputs.

  • Thank you, I think I was getting confused. Your concise answer cleared things up for me. Mar 19 '18 at 21:08

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