Looks like two people in this thread have suggested x = hearts. Why? I guess this is good when you have everything and partner is looking at equal length and nothing in both majors, but on a ton of hands partner will know which major you want or can at least guess it right (I bet he gets it right at least 50% of the time!). And it's not like your partner will automatically try to hit your suit if you don't double, he could easily have something like xxx xxx JT9x xxx and have a clear diamond lead. Also it's been stated before but the main benefit of doubling is not because you think they're going down, it's converting -600 into +200 or better. This happens very often! The downside is that your opps are unlimited and could easily be making a lot of tricks! Sometimes this will happen, but if you do the math, you really only need this to "work" around half the time, not to mention all the times it shouldn't work but your opponents misjudge (yes this is a common bridge theme, the more decisions you give your opponents the more they will get wrong, even if in theory they should get all of them right).