user-1: In Txin, there is a double hash of the previous transaction.
Why?  What purpose does that serve?  How is this data used later?

user11369: I believe it is one of the principles of Bitcoin, to make transactions you first have to have a valid transaction it stems from. If the previous transaction is not valid than the first transaction isn't valid as well.

-1: <p>In <a href="https://en.bitcoin.it/wiki/Transactions#general_format_.28inside_a_block.29_of_each_input_of_a_transaction_-_Txin" rel="nofollow"><code>Txin</code></a>, there is a double hash of the previous transaction.</p>

<p>Why?  What purpose does that serve?  How is this data used later?</p>
 11369: <p>I believe it is one of the principles of Bitcoin, to make transactions you first have to have a valid transaction it stems from. If the previous transaction is not valid than the first transaction isn't valid as well.</p>
