Everyone knows when to use the takeout double - at least when the bidding is at a low level.  Here is an easy one.  East opens I heart, and you (South as usual) hold:

 

(S) A K 5 4      (H) 6      (D) Q J 8 5      (C) A 5 4 2

 

You have a good hand with 14 high-card points (HCP) and a singleton.  If you had to open this hand, you would probably call 1 diamond and you might have difficulty describing this three-suited distribution.  East does you a favor by opening a heart, and now you can describe your hand with a single bid, the takeout double, saying: “Partner, I have substantial honors and I promise support for any suit you call“.  As takeouts go, this is about as good as it gets.  But suppose you hand is not quite as neat, like so:

 

(S) K Q 8      (H) 6 5 4      (D) Q J 10      (C) K Q 8 3

 

You have values to open (13 high-card points) but East is first and opens 1 heart.  You don’t have a suit to name, so what about a takeout double?  You can tolerate (barely) any suit partner returns, but you could easily end up in a 3-3 spade suit; so no, this is not a good takeout.

 

Well, if an opening hand does not necessarily make for a good takeout, then what does?  Here are some guidelines.  In each example, East opens 1heart.

 

(S) A 10 8 6      (H) 4      (D) K 7 6 5      (C) K 9 4 3

 

This is the absolute minimum - 10 HCP and four cards in every unbid suit.  You are sure to have a fit in partner’s longest suit and will be able to ruff some hearts.  It is this shortness in hearts that makes this a viable takeout.  Note the quality of your honors.

 

(S) A 10 8 6      (H) 4 3      (D) K Q 9      (C) K 9 4 3

 

Here you have two hearts, one less ruffing trick so you need more strength, at least two more honor points, preferably three.

 

(S) A 10 8 6      (H) 4 3 2      (D) K Q 9      (C) A K 4

 

Now we are getting into marginal territory.  You have three hearts.  This means no ruffing opportunities, and only three-card support in the minors, and you can bank on it that partner will return a minor.  Well not quite.  With a weak hand partner should prefer spades, which is the main reason you can get away with this takeout.  This hand has 16 HCP, so you might consider a 1NT overcall instead of a takeout double.