[Swammerdam]

Let's consider just auctions where RHO opens 1 of a suit after 0, 1 or 2 passes. What are the requirements for a Takeout Double?
I will describe my own standard. I solicit criticism.

My approach is to ENCOURAGE partner to participate by telling him right away I have support for his suit.
BUT, if I overcall instead of Doubling let him make the opposite inference: I'm probably uninterested in his suit.
(Some BBO'ers with excellent support for my overcalled suit insist on introducing some very mediocre suit of their own, instead of showing support. UGH!)

I distinguish two cases for the 1-level Take-out Double: (A) Three-suiter. (B) Strong hand (i.e. too good for simple overcall)

(A) Three-suiter with singleton in opener's suit.
With 4-4-4-1, double with any 10 hcp or a good-looking 9. 8 hcp may not be quite enough unless it's two Aces.
With 5-4-3-1, treat it similar to 4-4-4-1, perhaps a trifle more conservative. If you're doubling a minor and are 5-3 or 3-5 in the majors whether to double or overcall depends on suit quality.
With 5-4-4-0, treat it as 4-4-4-1. Don't reduce the hcp range: The Double promises defensive values.
(I read an ancient textbook which advised against doubling with a void -- If partner passes for penalties, you won't be able to lead a trump as directed.
I assume we will all agree that advice is VERY obsolete, if it ever had merit at all.)

Just moments ago -- (and one reason I'm starting this thread)-- my partner doubled a first-seat 2♥ opening with 4-0-4-5, two Queens and one King. Can we all agree that was WAY out of line?)

(A) "Three-suiter" with doubleton in opener's suit.
4-4-3-2 isn't really a "three-suiter" but I'll double as long as I have 4 cards in at least one of the unbid major(s).
The hcp requirement increases; instead of a goodish 9, I want a good 11.

With 5-3-3-2, the choice of double, overcall or pass depends on the 5-carder's suit quality and whether it's a major. Just a few days ago, I held
AKQ / K62 / K8752 / 42
RHO opened 1♣ in 3rd suit. I doubled. Show of hands, please -- Is the Double "correct"? I have good support for both majors, especially spades; and the Diamonds are down-right mediocre. Overcalling 1♦ never occurred to me. My partner, who labeled himself "Expert", told me "Overcall when you have five."
But in my style I WANT partner to bid a major and do NOT want him to throw in a nuisance raise of Diamonds at the 2- or 3-level with a doubleton.

With 4-3-3-3 I'll double if the 4-carder is a major and I have 12+ points outside the suit I'm doubling.

All of this is subject to context. A hand that doubles 1♣ may not want to double 1♠ and force partner to make a 2-bid. Doubles can be shaded slightly by a passed hand. Etc.

5-4-2-2 is obviously not a three-suiter. Unless it's a Strong Hand ((B) below) I never double with a doubleton in an unbid major and allow it in a minor only if that doubleton has an honor.

(B) So much for doubling with a "Three-suiter". The other case is a Strong hand, too good for an overcall.
The threshold -- What is the maximum overcall? What is the minimum to Double and then freely bid a new suit? -- may be a good question,
but for the purpose of this thread let me just say that
Even when I double intending to bid a suit on the next round, I will always prefer to have at least Qx in each unbid suit. I want partner to rely on me if he's forced to bid at a high level.

Comments?

[DavidKok]

I think your advice is somewhat old-fashioned, and leaves some points on the table in the long run. However, it's a very solid foundation.

The modern approach is to interfere aggressively, and to reserve a takeout double for all hands that wish to act but cannot take a more descriptive action. As a rule of thumb, if you can overcall cheaply in a five card suit this is preferred to doubling, while if you need to raise the level and/or conceal a 4cM in the process often a takeout double is preferred. However, the true answer is that no rule of thumb will suffice, as the flexible double changes meaning immensely depending on the situation. Lumping 0, 1 and 2 passes before the opening together is already quite inaccurate, but understandably it's good to have some common ground and then later introduce the nuances. The suit the opponents open also matters a great deal, which you pointed out. Be especially wary of having a doubleton in an unbid major, as partner might just raise that.

For me the example hand is a 1♦ overcall and not a double. However, I think in the broader community this is very much considered a style question, and I expect double to find majority support.

Ruling out 5422-type hands, especially (42)(52) hands after the opponents open the other major, goes against modern expert thinking but is a very playable style. Some people play Raptor overcalls and do not have to worry about this hand type so much.

When it comes to the 'too strong' hand type, it's very nice that you want to have at least Qx in each unbid suit. We all want things. What are you going to do when you don't hold those cards? Also, if you're going to double-then-bid anyway, what good is the doubleton in support for partner? I can see the value if you are worried about getting preempted and the bidding being too high to introduce your own suit, but even then you should strain to introduce your suit. The initial double to show strength is quite commital.