I was recently intending to give a lesson on opening leads to a very keen couple, but before we started they asked me about a recent disaster they'd had. The auction had gone 1NT-2♦ - X - 2♥, P - P - X - P, P - P, the 2♦ bid being a transfer. The contract made, and the inevitable recriminations had started. One careless comment from me could have destroyed a happy marriage!
Whilst thinking "The 1NT bid was impeccable, and the final pass would be hard to quarrel with, but as for the three calls in between...," I prevaricated with "This is actually a difficult sequence." Clearly the carefully prepared opening leads lesson would have to wait for another day!
I soon ascertained there were two problems - for one thing they did not know what their doubles meant, and secondly they didn't know when they should be trying to penalise the opponents.
So when should you aim to penalise opponents? What people need is an easy rule of thumb, a bit like deciding when to open 1NT. For that you would no doubt use point count, and the shape of the hand. In other situations you might well use my favourite hand evaluation method (for my opponents) - losing trick count.
To decide whether to double the opponents you need two things - point count, and number of trumps. My simple rule is that if you have the balance of the points, then double the opponents at the two level if you and your partner have at least six trumps between you. Don't double if you have four or fewer, and usually don't with five trumps. If they are at the three level, reduce the number of trumps required to double by one, and by another one if they are at the four level.
This rule relies on the idea that generally speaking, the greater the trump fit, the more tricks people make. It obviously depends a bit on the hand - with AKQJxxxx xxx x x if the auction starts 1NT-3♥, they may only have an eight card fit, but with such a fine source of tricks, this is the time to ignore the rule and simply bid 4♠. However, with a 3334 11 count, just double them. If it's wrong, save your marriage by blaming me.
If the auction started 1NT-2♥, you now have very little room to establish how many trumps you have - you are inevitably going to have to guess a bit. However, if people are kindly playing transfers for you, it gives you the chance to have a dialogue with partner. With ten points or more, you know you have the balance of the points, and you can proceed as follows over 2♦.
With one trump or none, bid 2♥ - showing a good hand but no desire to take a penalty. With four or more trumps, pass 2♦ and hope to get another chance next round to double 2♥ - which is unconditionally for penalties.
With two or three trumps, start off with a double of 2♦. If your partner has four trumps, he will double 2♥, knowing that you have six trumps between you. If he has three trumps, he will pass, letting you decide whether to double or not, which you will do with three trumps (with only two trumps you bid a suit, or cuebid 3♥, or perhaps bid no trumps). If opener has only two trumps, he bids a suit, or passes, and then pulls if his partner doubles - knowing that opponents have an eight card fit he will not defend 2♥ doubled.