Conceptual question ?

[Stibit]

I am getting up to speed but remain confused about one thing.

I have a bitcoin both on my machine in a local Wallet and in my Blockchain wallet. Its the same BTC.

Now I have added another BTC to my local Wallet which shows 2BTC balance (including the original) - which can both apparently be sent to my Blockchain wallet.

If I do that, and presumably blockchain realises that one of these is already there so gives a correct balance of 2BTC

If I then spend one BTC from my local Wallet - how and when does my blockchain wallet update itself with my new, reduced balance ?

I think its the concept of the same BTC being in various 'places' that is messing with my head. Whats to stop me spending the local coins and then spending the blockchain coins on something else before blockchain gets synced ?

Apologies if I have missed the point here but if someone would be kind enough to help me get that straight....

[Panoramix]

Your coins only exist in the blockchain (which is like a ledger). Your wallets (local and web) only hold the keys for for accessing/spending your coins. Your local client and web client are just two ways to look at the same blockchain. There can be a difference in the time that you see your transactions appear in your client due to sync settings of that client.

[Stibit]

Thanks - that does help. How does the system stop coins being spent twice prior to network sync ?

[Gabi]

What "network sync?" When you send a transaction every node will receive it and a miner will put it in a block. If you try a doublespend, it will be refused by everyone.

Also please do not confuse blockchain with the website blockchain.info (yay, so much idiot to name a website like an important part of bitcoin, so everyone is confused...)

[DannyHamilton]

How does the system stop coins being spent twice prior to network sync ?

It doesn't.  If you have both wallets set up to be accessing the exact same coins, it is entirely possible for you to accidentally try and send those coins to two different places, one from each wallet.  This is what a "double spend" attempt looks like. Then it is possible for some miners to be working on one of those transactions, and other miners to be working on the other transaction.  Whichever miner solves the block first will determine which transaction is confirmed.  The other transaction will remain unconfirmed. All the peers in the network will eventually forget about the unconfirmed transaction.  If your local Bitcoin-Qt is the wallet that sent that unconfirmable transaction, then it will probably get into a messed up state where it will display the wrong balance and possibly have issues attempting to create new transactions in the future.  For this reason it is a bad idea to be trying to access the same coins from two wallets that aren't designed to work together.

[DannyHamilton]

(yay, so much idiot to name a website like an important part of bitcoin, so everyone is confused...)

Like Coinbase?

[wachtwoord]

What "network sync?" When you send a transaction every node will receive it and a miner will put it in a block. If you try a doublespend, it will be refused by everyone.

Also please do not confuse blockchain with the website blockchain.info (yay, so much idiot to name a website like an important part of bitcoin, so everyone is confused...)

Well the first (and to me still primary) goal of Blockchain.info is to display the Blockchain and related statistics. So to me the name works very well. They probably should have used a different name for the wallet though.