Just what we need -- another rule! Actually, this one is a bit archaic, but has some merit. It concerns preempting and vulnerability.
Unfavorable - "2"
I am a big believer in observing the vulnerability when preempting. If "Unfavorable" (Vulnerable against Not Vulnerable), I recommend being sound and cautious. The Rule of 2,3,4 states that when at this unfavorable vulnerability, you should be within 2 tricks of your bid. So, let's say you have this hand, vulnerable against not:
K Q J 10 9 5
J 5 2
You have 5 tricks in your hand, so should not preempt with 2. You are not within "2" of your bid. But, change it to:
K Q J 10 9 5
K 5 2
and you have 6 tricks (you count a supported king as a trick) and can open 2.
A K Q 10 8 7 5
6 5 4
you can open 3 at unfavorable vulnerability, because you are within "2" of your bid (you have 7 of the 9 tricks you are contracting for).This is good advice not just for safety (avoiding -1,400, for example), but for describing your hand to partner. If he can count on your being roughly within two of your bid when unfavorable, he can often put you in game based on that knowledge.
What about other vulnerabilities?
Equal Vulnerability - "3"
"Equal" means both sides are not vulnerable or both sides are vulnerable. In other words, each partnership (N-S and E-W) are at the same vulnerability. In this case, the Rule of 2,3,4 suggest that you be within 3 tricks of your preempt.
So, with neither side vulnerable, you can open 2 with:
A 7 6
Q J 10 9 8 5
You expect 4 heart tricks and the A for 5 tricks, thus preempt on the 2-level (you are within "3" of 8 tricks).
Favorable Vulnerability- "4"
As my good friend and fellow teacher, Roberta Salob likes to say: "Unhook your bra, baby!" Or, as my longtime partner and friend Marty Bergen says: "Favorable vulnerability is not bridge." In other words -- bid like crazy! The "Rule" says to be within 4 tricks of your bid, so you can open 2 with:
Q J 10 8 7 6
5 4 3
5 4 3
That (optimistically) looks like 4 tricks (all in spades), so you are within 4 of your contract. What if your convention card says "5-11 HCP" for a weak 2-bid? Not a big deal. Maybe you should write "3-11," -- but it is understood that the range should be affected by vulnerability.
What do I think of the Rule of 2,3,4?
It is a reasonable guideline, but can't be taken too seriously. First of all, everyone has their own style and aggression-comfort level. Some might play the Rule of 3,4,5 (wild preemptors) and others might be closer to 1,2,3. Personally, I prefer to look more at my own vulnerability. When I am vulnerable, I tend to be within 2 tricks (roughly) of my bid. When not vulnerable, I am more lax.
I am a big believer in suit quality. If I have a suit of J98764, I am not likely to preempt -- no matter how many tricks I think my hand is worth. I want my partner to be able to lead my suit. I also live in fear of: Pass-Pass-Double-All Pass. When I have a decent suit, this isn't likely to happen. My general guideline is to preempt on the two-level with a 6-card suit and the three-level with a 7-card suit.
Position also is a big factor. In second seat, I am always sound (even not vulnerable). When RHO has passed, there is a good chance the hand belongs to our side, so preempting should be well-defined and disciplined. On the other hand, in 3rd seat (when my partner has passed), I know it isn't our hand, so I take more liberties when preempting. As dealer, I am somewhat "in the middle--normal." Summary of position: 1st seat-normal. 2nd seat-sound. 3rd-seat-aggressive.
Also note that you can't always calculate how many tricks you have. Who is to say what K109765 counts as? How about a suit such as AQ8764?
One other popular way to evaluate opening bids is to count losers. I'm not a big fan of losing trick counts, particularly for opening bids, since they often wind up either confusing us or miscalculating the value of the hand.
Lastly, what about "two of the top three honors, or three of the top five?" well, that's a useful stand-in for suit quality (although it ignores 9s and 8s), but I hope most will find a more nuanced approach to preempts.
The Rule of 2,3,4 is a nice guideline to be aware of. Try to base your preempts on vulnerability, suit quality and position at the table.
Updated: July 2021