You want to buy MSFT at $24 or better. The stock is currently trading at $24.12. So you wait. And wait. And you wonder, "will this trade ever happen?" (It may not.) But then there's a big market down-draft day and the stock kisses $23.99 for 20 seconds. If your broker is good, he'll pass along that extra penny to you - i.e. buy the shares for a total cost of $23.99 to you, not $24. But at worst, you'd pay $24.