Protective Double

A protective double is NOT a Take-Out Double

It is made on balancing values - the presumed combined total held by you and your previously passed partner.
You may ask what is this combined value?
First consider that your LHO opened 1 of a suit. Presume he has good opening of say 13HCP. He may have a King value less or more - just be aware. Also the probability he has only 10HCP is actually far more likely than he has 16HCP!
Your RHO has not responded. Give him 0-5 HCP. Say 3 for the purposes of this calculation.
If these suppositions are correct then your combined count with your partner (who has not spoken either) is 24 +- 5 HCP!
Therefore an effort to speak is imperative and even a game cannot be excluded!
Be aware too that if partner has good values even opening values or better, that he may not be able to have overcalled or doubled due to holding significant values in the opponents suit and he may just be praying that you reopen with a double!
So be prepared to make a PD from 10HCP even on a flattish hand. With a good suit lower that limit to 8 HCP.
Your partner is expected to bid his suit even if his RHO passes unless he is able to convert your double into a penalty double - don't be surprised it does happen this way!
Don't forget you did have other options than double - principally holding 2 suits 4-4 or better: by bidding the lower ranking cyclically: e.g. 1H P P 1S show Clubs too. Also remember our unusual 1NT bid to show the 2 lowest unbid suits 5-5 or better - weak or strong or a pre-emptive bid in the lowest unbid minor etc.

If your LHO has opened with a weak 2 bid then the mathematics is different: LHO may have 5-10 and your passed RHO may have +/- 8HCP without a fit. Now it is even more certain you may have a game! Aggressive action is surely required - partner may not have a good bid available to him with as much as 15 HCP! He will be relying on you for some action! He has Rubinsohl available to him  with a good 6+ card suit, also a T-O double and even a simple 2-level overcall e.g. 2S over a weak 2H opening with even a good 5 card suit. However over 2S very limited options are available to him just Rubinsohl & Double & PASS.
On a few occasions your LHO may have 18 HCP and may be rather grateful you doubled rather than PASS! He may then PASS for 'reverse penalties'! (partner to take required action if opponent redoubles for game!)

When LHO opens 1NT his point range is known within narrow limits and we also get a fairly clear view of his partners upper limit of say 10 when he PASSES opposite a 12-14 NT. More caution is required with a 1NT opening than a weak2 or simple opening. Assume opponents have a combimned range 18-24. Make the assumption of 21HCP +/- a King value which also gives then in all probability the balance of the HCP. However one should still be prepared to make a PD with 10HCP if holding a good suit. Remember you have alternatives to double such as 2C or 2D when holding the suit and a higher suit & 2H & 2S with a 5+ card suit. If these are unavailable this leaves you with a choice between PASS and a PD. Partner will be aware of your options and hearing a PD will expect a flattish hand or a 5 card minor suit.

A PD requests action from your partner as follows:
(i) To choose his longest/best suit with Less than opening values - Protective Doubler is likely to have a fairly flat hand - therefore extending some support in all suits - with 2 suits to bid the lower ranking first.
(ii) PASS ONLY for penalties 11+ HCP with a suitable hand.

This is a dramatic reduction from the 15+ HCP requirement for a PD as recommended in ACOL. Lower requirements are being met more and more frequently in this modern aggressive bridge world! I have had a 10+ PD bid against me, effectively too, in a good field! Also remember to alert our PD following a weak 1NT opening.

In the sequence 1NT P P * P ? It is known that partner has not been able to show 2-suits or a good 5+ Spade suit or use Rubinsohl. Therefore partner is very likely to hold a 5 card suit (admittedly just a 4 card in a 4333 hand) . Now the protective double will be soliciting which suit is held. PASS is NOT the usual response unless willing to pursue penalties. 

More caution is required with unfavourable vulnerability - especially when vulnerable against non-vulnerable when opponents will be overly keen to score 200!

If opponents redouble,  partner must take it out as 1NT ** is a game call.

So partner's  possible responses to a protective double:
               Converts to penalties
Suit bid             Natural, Preference given to any 5 card suit or lowest 4 card suit (which he would not have been strong enough to use CDH at his first bid.

A protective double should not be made over a 1NT opening with a single suited hand with a 5 card Major. A direct take-out can be made.
With a 6+-card suit there are 2 options:
A) Bid it directly at the 3-level if you have a 7+ card pre-empt
B) With a good 6 card suit: First make a PD and then bid your suit over partners response if you are short in his suit

A protective double should not be made over a suit opening if holding TWO 4+ card suits - instead bid the lower cyclically & of course PASS with nothing to say