I am a newbie and have what may be a fairly basic question. Suppose the stock of a large heavily traded company (e.g. GE, CAT, AAPL, etc.) closes at 4 pm on Monday at a price of $20.00 per share. Suppose that I place an after-hours/extended hours limit order to buy at $20.50. Then suppose the stock opens at $21.00 the next morning at 9 a.m. What is the probability that my buy order will be executed within +- $0.10 of my limit order price overnight, sometime between the close and the open of the regular market? Put a different way: what percentage of such orders will remain unexecuted the next morning? Is the outcome fairly predictable, or so variable from stock to stock and ECN to ECN that the question itself is silly?