How do you specify the From address when you send a payment

[grchris]

Hi there,

I have never used bitcoin before and I am wondering how you can specify the From address when you sent a payment.
In the Sent Tab of bitcoin there is the Pay To box that you enter the address you want to sent payment to, a Label and an Amount box.
There is no place to enter the From address.  So if you have two incoming addresses in your wallet how would you know which one was used to sent the payment from?

Thanks and sorry if this is a stupid question.

[CIYAM]

In the standard client there is no way to do this (although you can use the Coin Control patch to do so*).

The software automatically chooses UTXOs and creates a "change address" for you if they do not equal the exact amount you are sending.

So let's say you have 3 UTXO's as follows:

1Addr1 0.2
1Addr2 0.2
1Addr1 0.2

and you want to send 0.3 to 1DestAddr then it will use two of the three UTXO's (which could be both 1Addr1 ones or one 1Addr1 and 1Addr2 one) then create a new address (1Addr3 let's say) to send the 0.1 change back to yourself.

*Note that if it choose to pick 1Addr1 and 1Addr2 UTXO's then there are *two* so called "from" addresses (i.e. the balance of an address is actually just the total of all UTXOs for it).

[grchris]

Thank you so much for the quick reply!

So if I have two incoming addresses and only one of them has any coins is it safe to assume that bitcoin will use the one with coins by default?

The reason I am asking is because I would like to send coins to buy the USB miner in this thread

https://bitcointalk.org/index.php?topic=264633.0

but once I sent the coins I am supposed to post a msg in the thread like

grchris; 1; 0.55; XXXXXXXXXXXXXXXXXXXXXXXXXXX

where XXXXXXXXXXXXXXXXXXXXXXXXXXX is my from address (not the pay to) that I used to sent the coins.


Finally I am supposed to send a signed msg using the From XXXXXXXXXXXXXXXXXXXXXXXXXXX address to send him my postal address.

I am a little nervous as to if I will be able to prove that I was the one that really sent the coins if I can not identify which From address was used for the payment.
So in a sense I do not really need to know and select the From address in advance but I do need to be able to identify which address was used after I sent the payment so I can communicate it to the seller. 

Do I make any sense? Am I using the wrong client?

Thanks a million again for your help.  I wish I could post this question to the above thread but I am a newbie and I can not post in that section.

[astutiumRob]

look at the transaction on the blockchain to see which address(es) it sent the money from

[grchris]

Thank you.  I think I got it.