With a hand like (s) Axxx (h) KQxx (d) Kxxx (c) x, I would gladly make a takeout double over one club. That's because my partner would have a choice of three suits to bid at the one level.
Change the hand so that the (singleton and bid) were in spades: (s)x (h) KQxx (d) Kxxx (c)Axxx. Now, if I make a takeout double, partner would have to respond in a suit at the TWO level, possibly with as little as (s) xxxx (h) xxx (d) xxx (c) xxx.
In this situation, I would double one spade (reluctantly) if not vulnerable versus vulnerable, and pass otherwise.
Edit: Opposite a "Yarborough," I expect my hand to be worth five tricks on offense. Thus, I am willing for my partner to contract for seven tricks but not eight, thereby "overbidding" by two tricks, but not three, with equal vulnerability, against the opponents' presumed game.
Does one need to be more careful about takeout doubles over a spade bid than a minor suit? That is to say, is my reluctance to risk down three well founded, meaning that I have "drawn the line" in a good place?
This question is for rubber bridge; duplicate is a different animal.