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I have a desktop, laptop, tablet, and phone. I want to be able to send and receive payments from all these devices.

How do I go about doing that? Do I have to create a unique wallet on each computer?

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    This is a good place to start bitcoin.org/en/choose-your-wallet - I would select MultiBit (PC) and Mycelium (Android) and my answer how to import private keys – Jonas Oestman Nov 22 '13 at 19:51
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    +1 Why don't you turn that comment into an answer along the lines of create one wallet on one device and then import it to the other devices? Note that only for sending this is required; for receiving, you do not need a single device or client after all (and to check for receipt of a payment, you only need your Bitcoin address, not the wallet). – pyramids Nov 23 '13 at 11:10
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Copying the same wallet.dat to two different computers will initially seem to work, but if you continue to use bitcoin-qt to send bitcoin, the two copies will get out of sync.

The reason for this is the automatic use of "change" addresses. Each time you send an amount less than you originally received, there is "change" that must be returned to you. The developers of bitcoin-qt considered this to be a privacy weakness, as it would make it easier to tell for sure how much was paid and to whom. To make it more private, they generate a new random "change" address in your wallet and send back your change to that new address. To a third party looking at the block chain, it is hard to tell who holds the change address or who was the actual recipient of the transaction.

The reality is that the use of change addresses does little to improve privacy, but sets up new users for a "big surprise" the first time they send a small amount and see their wallet balance temporarily go to zero prior to confirmation of the change. It also makes it unsafe to copy the same wallet to multiple computers.

There are other wallets that give the user control over use of change addresses and that also make the change addresses visible and therefore more understandable.

Another area where other wallets can help is in the generation of new addresses. The original bitcoin-qt wallet generates new addresses randomly. It tries to keep a pool of pre-generated addresses, so that if you copy a wallet it will seem to generate the same new addresses as the original, but eventually the pool of keys will be used up and after that the two copies of the wallet will start to have different contents. The Electrum guys solved this by making the whole wallet "deterministic". That means that all future keys are determined completely by the "seed" and are not truly random. Because of this, you can restore the same wallet on multiple machines and all copies will forever remain in sync.

There are plans to add this sort of functionality to the main bitcoin-qt wallet, but until that is done, alternate wallets are needed if you want to share between computers. Electrum is not the only wallet that provides all these features, but it is the one I am most familiar with. See electrum.org.

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You simply locate the wallet.dat file and copy it to each client. The wallet file is simply a KEY to your coins, and not actually the coins themselves...

https://en.bitcoin.it/wiki/Data_directory

In my opinion, the word 'wallet' needs to be replaced with something that doesn't confuse everyone. Good luck!

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    be careful of change addresses. if you copy the same wallet.dat to two computers your original balances will copy over, but new transactions will make new random change addresses that will not be present on both copies. it would be safer to use a deterministic wallet, like Electrum. – hsmiths Dec 7 '13 at 20:12
  • @hsmiths, could you please convert this into an answer with further details? – oshirowanen Dec 15 '13 at 20:07
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You can do so by exporting your wallet including your private key to those different clients which is rather easy if you use the Bitcoin-qt client, or alternatively you can use a hosted wallet service such as Coinbase or Blockchain Wallet. Keep in mind that a hosted service can potentially be less secure than a wallet on your computer. This guide on the Bitcoin website can help you decide what wallet client is right for you

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