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While browsing the blockchain I noticed that there are a lot of transactions that send bitcoins to the sender (most part of the transaction) and a tiny fraction to another address.
E.g. this transaction sends more than 99% to the sender and the rest to another address.
What is the purpose of those kind of transactions?
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An output of a previous transaction can only be spent wholly. So if you have 3BTC and want to send 0.5 to someone, you make a transaction that sends the 0.5 and then 2.5 back to your own address. This is called "change", just like you get change when paying in the supermarket, except in this case you give the change to yourself so you don't need to worry about the other person running of with it.
Note that this change address is supposed to be a brand new address (owned by yourself) and NOT the same address. Address re-use is bad for privacy: as you've just discovered you can in those transactions easily determine which part of the transaction was change and which was actually sent to some other person.
Hi Jannes. This is a very common question that gets asked a lot, and in such cases, it is better to mark the question as a duplicate rather than post a new answer. If you have something to add to existing answers of the previous question, post a new answer there rather than here. That way information is kept in a single accessible place, rather than being spread across dozens of similar questions. – Nate Eldredge – 2016-09-08T13:39:15.347
@NateEldredge You're right, I was going to link to other "change" questions, but I was on mobile earlier. It seems good to me to have a short explanation here and then link further for details. Anyone searching for an answer but not knowing the term "change" could find this topic before another. – Jannes – 2016-09-08T13:46:07.287
@Murch Feel free to mark this question as duplicate, but I think my question is a little bit different than the one you mention since in my case I didn't understand in the first place that an output must be spent completely. – Stringer – 2016-09-08T16:30:35.277
For me the question would rather be whether the answers on the linked topic completely answer your question. I should have written "related" instead of "duplicate". That would have been more accurate. ;) – Murch – 2016-09-09T12:26:25.703
Sure, I just marked it as duplicate. – Stringer – 2016-09-09T13:15:25.637