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Comment by 3dfan | Hacker News Reader

The fork that is on the horizon looks like this:

    On chain A blocks bigger then 1mb will be valid
    On chain B blocks bigger then 1mb will be invalid

Now when you sign a transaction and broadcast it, it will get executed on both chains.

Unless the transaction has coins in one of its inputs that already went through a block bigger then 1mb. Then the transaction is only valid in chain A. This way you can double the value of a coins that are spendable on both chains. Because now you can sell them once on chain A and once on chain B.

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Right. But economically speaking, if exchanges started allowing you to trade both coins against each other, then supply has doubled while total Bitcoin demand remained constant. So the price would tank to at least 50% of its previous level on each side of the fork to reflect the new equilibrium.

Anyway, I think this is extremely unlikely to happen, because:

1) It requires 50% of miners to cooperate to even create a >1MB chain

2) Anyone using the old chain would be insecure, due to lack of mining power making it susceptible to a 51% attack.

3) If that many miners were on board, it's unlikely that existing core developers would opt to continue working on an insure fork rather than admit defeat.

4) If that many miners were on board, it's unlikely that exchanges would opt to confuse users by letting them separately trade two different types of Bitcoin, rather than just switching to the more secure, more popular chain.

I'm presuming 3dfan is assuming some kind of split in merchant/exchange acceptance, I agree this is less likely, though. However, in a hardfork, you can be sure there will be some kind of forkA/forkB exchange site that will emerge (not at a 1-to-1 exchange rate, of course).

This is correct. The simplest way to do this would be to include in your transaction a spend deriving from the coinbase block reward after the fork.

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