Sometimes partner makes a double for takeout, but you decide not to bid your longest suit because that’s the suit the opponent opened!
Have a look at this hand:
South has such extreme diamond length, it’s best to pass, converting what was meant to be a takeout double into a penalty double. The lead should be a trump, as the idea is to draw declarers’ trumps because the defenders have so many of them. If defended properly, the result will be 800 to North South. Ouch!
97.J42.KJT965.A8 KQT8.KQT9.3.KJT7 J653.85.72.Q6543 A42.A763.AQ84.92