I’m hung up on what exactly is meant by “reverse transaction” in double-spend.
Michael Maxwell

Hi Michael, thanks for your question. No, John doesn’t send anything to Mary, it’s Mary that tries to trick the system performing what is called “double spend attack”. Here a short overview of how it works. Let’s say Mary sends 10 Bitcoin to John (“M → J 10" in fig. 8) as a form of payment for a product he sends her. She executes such transaction on the blockchain from her wallet. If Mary could later generate a longer chain tail with a transaction that points the 10 Bitcoin back to her (“M → M 10” in fig. 8), what I called “reverse transaction”, she could prevent John from receiving the bitcoin as the initial “M → J 10” transaction would not be considered valid from the network nodes. Hope this helps. You can find more info on Double Spend Attack here: bit.ly/2o9A85X

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