This real deal was dealt by Tracey Yarboro.

Vul:None Dlr: South	A1083 8653 QJ K109
KJ972 AJ42 3 J87		Q54 KQ10 74 A6543
	6 97 AK1098652 Q2

West	North	East	South
			4
All Pass

South has a beautiful 8-card suit, but not enough HCP to open 1. However, South shouldn’t pass. She can open with a preempt in diamonds. At what level?

With a 6-card suit (move two of the small diamonds elsewhere), the preempt would be on the 2-level. With a 7-card suit (move one small diamond elsewhere), the opening would be on the 3-level. With the actual 8-card suit, South should preempt on the 4-level via an opening 4 bid. This doesn’t mean South expects to take 10 tricks. A reasonable guideline for a preempt at this vulnerability would be to have roughly 7 or 8 expected tricks in your hand.

What should West do? An overcall on the 4-level would require much more strength and a better suit as well. A double would be “takeout,” showing at least opening-bid strength and the other 3 suits. It is tempting for West to come in, but the level is just too high. Had the opening bid been only 1, West could come in with a 1 overcall.

After West reluctantly passes, nobody else has anything to say. The 4 opening will actually be the only bid of the auction.

Whatever West leads, declarer will have an easy 10 tricks. All that is lost are 2 hearts and the CA. Plus 130 for North-South.

What if there was no preempt, or East-West got into the auction? If South passes as the dealer, the deal actually could get passed out! What if East-West find their 5-3 spade fit? They might make 9 tricks. The play is tricky, but you’ll have to take my word for it that only a club lead would set a 3 contract.

What if East-West play in clubs? Maybe West makes an aggressive (that’s the nicest way I can put it) takeout double of 4 and East pulls to 5 (not that passing would be any better). To pull a 4-level takeout double, you should try to have some decent distribution. East’s hand is fairly flat, and he would probably leave in a double of 4 (the score for that contract making is 510). A takeout to 5 could be disastrous (especially if doubled). South could lead the singleton spade and look what happens. North wins the A and gives South a spade ruff. With clever signaling (using something called a suit-preference signal), South might know to underlead his diamond honors to reach partner for a second spade ruff. North-South have the first 4 tricks and North still has 2 natural trump tricks. East-West could be held to only 7 tricks in a club contract. That would be a score of 800 to North-South if they double and defend perfectly.

Lesson Points:
1) A 4-level preempt typically shows an 8-card suit.
2) A double of a 4-level preempt is not for penalty—but requires more than West holds in this diagram.
3) Take out a double of a 4-level preempt only with reasonable distribution and a convenient bid to make.

Related:

Larry teaches bidding over the opponents' preempts