You open 1♦ and after a pass by LHO, Partner bids 1♥ .

RHO now makes a bid of either1♠ ,1NT, double or 2♣ .

You have 3-card support for partner, and do not know whether to support him or not. Partner will take you for 4-card support if you raise him.

The answer is to play support doubles. When it is unclear if partner has a 4 or 5 card suit, the double explicitly shows exactly 3-card support. Note it says nothing about the strength of your hand; it just shows how many cards you have in partner’s suit.

So, after 1♠  , 1NT, or 2♣ , a double would be the support double.

After a double, a redouble would show the 3-card support again.

However if you open 1♠  and partner responds 2♣ , after any bid by the opposition, you will bid normally. The reason being is that partner has shown a 5-card suit and it is not necessary to have 4-card support. You can raise with only 3.

You ONLY use Support Doubles if it is unclear if partner has a 4 or 5 card suit