16 vs X
"Lawrence Revere" dealt with this in the "fine points of basic strategy" section of his book, Playing Blackjack as a Business.
The key card here is the Six. The more Sixes you have in your hand, the fewer are available for the dealer to have under his ten-point up-card.
Thus, if you hold, say, a 7-3-3-2-A vs X, the proportion of Sixes available for the dealer to have down is increased significantly; and the call is so close this supply of Sixes is enough to warrant standing in hopes of a dealer break.
Conversely, if you hold something like 6-3-6-A, the proportion of Sixes is diminished enough to warrant hitting, as the probability of the dealer having a Six down is significantly diminished and his probability of having a pat hand is thus increased.
The Fours and Fives also have an effect in this situation, but much less than the effect of the Six; so that's the card to be mindful of.
