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change address is an additional address inside a wallet, which u can create with a raw transaction with 2 outputs.

What I want to know is how does this change address interact with the (for a lack of words) "original" address, when sending money back to the "original" address.

Does the change address have a pre-written script which sends BTC back to "original" address?

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  • by 2 outputs, do you mean you are sending to 2 real people separately in the same single transaction? or is the 2nd output your way of saying the change address that goes back to you?
    – user610620
    Commented Dec 9, 2022 at 5:21

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There is no relation at all between the two addresses, except that they're owned by the same person/wallet.

At the protocol level, "change" does not exist. It's just another output (with a corresponding address), indistinguishable from normal "payment" outputs. It is your wallet that's creating it: it needs a place to send the remainder of a spent UTXO of yours that you don't actually want to transfer, so it creates a new address of its own (just like you'd do when receiving funds), and sends the remainder there. And it stays there, until there is a need to spend those funds. It could send them back to the same address, but this would be a serious privacy leak (it lets people distinguish change from payment), and have no cost advantage.

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  • If my wallet is creating the "change address", how do I know or have assurance that I own the bitcoin that is inside the change address? All I started with was the sending address, which I know I have the private key for. The change address, however, I do not have a 2nd private key for.
    – user610620
    Commented Dec 9, 2022 at 5:19
  • You obviously have to trust the software you're using to do the right thing. Your wallet, if functioning properly, does have the private key for the change output it itself creates.
    – Pieter Wuille
    Commented Dec 9, 2022 at 13:28
  • How would i then next send Bitcoin from the change address to someone, instead of from my original wallet?
    – user610620
    Commented Dec 9, 2022 at 14:36
  • Your wallet manages all the addresses, both receive addresses and change addresses, and will show you as balance the sum of all funds it can control across all of them. You don't need to care about what is in which.
    – Pieter Wuille
    Commented Dec 9, 2022 at 14:47
  • What is the maximum number of change addresses that a wallet can generate?
    – user610620
    Commented Dec 9, 2022 at 15:26
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It doesn't interact with the "original address". It's just another address in the wallet. The wallet keeps track of all of its addresses, receiving and change.

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  • How to get a list of change addresses my "wallet keeps track of", say on a Ledger?
    – user610620
    Commented Dec 9, 2022 at 14:38

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