Now instead, let's suppose the dealer's got that 10 up and you're dealt a 7/5. So you hit and catch a 4 to make 16. If you think this 7/5/4 is the same thing as a 10/6, think again. When you held the 10/6, two cards that would bust you if you hit (the 10 and 6) were out of play. But when you hold 7/5/4, the 10 and 6 are still lurking somewhere while that 5 in your hand (one of your potential 21s) and the 4 (a potential 20) are now dead. On a hand where the right play is such a close call, this is enough to turn a correct hit into a correct stand. Mathematical and computer analysis studies have backed this up.
The Rule of 45: In fact, your 16 doesn't even have to contain both a 4 and a 5 to make standing the right play against a 10. Either of them, the 4 or the 5 is enough to do the trick. If you have 9/4/3 or 8/5/3 or 9/5/2, etc. -- any 16 that contains a 4 or a 5, stand against a dealer's 10. Doing that, you'll pull an extra hand, and a terrible hand at that, out of the fire every once in a while. But just hit any other kind of 16, such as 6/8/2 or 3/10/3 or 8/7/A. That's known as the "Rule of 45" and it takes the game one small step beyond a basic strategy chart.