So, I've been thinking about when we can safely make a penalty double.
1. After partner opens/overcalls 1NT, your doubles are penalties.
If partner opens 1NT, then you know he's got points and a few cards in each suit.
|♠ K 9 8|
|♥ Q 6 4 2|
|♦ A 4 2|
|♣ J 3 2|
Time to double for penalties. If partner has overcalled 1NT it's the same, but he has an even stronger hand, so you need less. For example:
With the hand below, South can also double for penalties.
|♠ J T 9 4|
|♥ A 4 3|
|♦ Q T 4|
|♣ 5 4 3|
2. If partner's already doubled for penalties, your doubles are penalties.
South can bid, if he has a very long suit and doesn't want to defend. A balanced hand with a few points Should double:
|♠ A J 2|
|♥ Q T 3|
|♦ 8 7 6 2|
|♣ T 3 2|
3. After partner preempts, your doubles are penalties.
Partner has described his hand well with the preempt, so the only possible meaning for your double is penalties. You need a really good hand here though, as partner doesn't have much.
|♠ Q J 9 8 3|
|♦ A K 3|
|♣ A Q 3|
and can double for penalties. I was on the wrong end of a double like this against Jake, in the second hand here: Weighing In.