Firstly, I assume that you won't see a new bid/ask placed in the market when you enter a spread order, because you have a limit for the whole order and not for each leg. So, are spread orders posted somewhere? Does a floor trader just keep the order written down until he can fill it? How does the broker keep track of the order and guarantee both legs filled at the same time? Are spread orders executed against other orders in the market, or only against other spreads, or does the broker do it in-house?