Sunday, January 31, 2010

Low Straight Blockers

When betting on a high pair in Seven-Card Stud High-Low, against a player showing a bunch of scary-looking low-cards, the possibility of being scooped by a low straight must be considered. Conversely, if you have a bunch of low-cards, how likely is it that your hand will also fill out into a low straight? The chances of a bunch of low-cards turning into a low straight depend on the availability of the "blockers." That is the location of cards that are necessary for a low straight to form. If the needed cards are dead, the low straight is blocked, and the relative value of the high-hand goes way up in relation to the low-hand.

That means that when reading the board, you should look for exposed cards that block the completion of low straights. For example, if you started with 2, 3, 4, and picked up a 6 on Fourth Street, be on the lookout for the 5s. If three of the 5s are exposed on the board, your hand has gone way down in value. Your straight potential is down to just one out. You are essentially drawing for just one half the pot, which is a violation of the most fundamental principle of high-low poker, that you should always play to scoop.

Knowing the blockers is also important if you are going high. Suppose your hand is (4, 4,) 4, J and Bob raises with (X, X) 3, 5. You see the remaining 4 in Alice's hand. Your draw for quads might be dead, but more importantly; Bob's draw for any kind of low straight is completely dead. He will not hit a wheel, or even an 8-high straight. You can re-raise Bob all you want, because he will not beat you with a straight.

Here is a table of blockers for low straights (8-high or less):

Dead Rank straights blocked straights possible
A 5-high 6-high, 7-high, 8-high
2 5-high, 6-high 7-high, 8-high
3 5-high, 6-high, 7-high 8-high
4 5-high, 6-high, 7-high, 8-high none
5 5-high, 6-high, 7-high, 8-high none
6 6-high, 7-high, 8-high 5-high
7 7-high, 8-high 5-high, 6-high
8 8-high 5-high, 6-high, 7-high

It is important to note in the above table the critical role of the 4s and 5s. If either rank is dead, no low straights (8-high or less) are possible. If the 4s are dead the minimum allowed straight is a 9-high, and for the 5s the minimum is a 10-high. Players with exposed low cards (8s and less) are less likely to reach straights that high.

Also note from the table that these two-card combinations that will block all low straights.

A, 6
2, 6
3, 6
2, 7
3, 7
3, 8

In other words, if any single wheel card plus the 6s are dead, no low straights are possible. Dead 2s or dead 3s combined with dead 7s block all low straights. Dead 3s and 8s block all low straights.

Keeping track of blockers is especially important on later streets in determining hand values. Two 5s might be gone on by Fourth Street, but if all are dead by Sixth Street, that is useful information for deciding whether to value bet on the end.