These days there are many varieties of what to do when an opponent doubles you for penalties after you have opened 1NT. Unlike the opponent’s double of a suit for take-out, their double of a 1NT opening bid is usually for penalties. NOTE : It should be alerted if it is NOT for penalties.
After 1NT is doubled, initially players are taught that with a weak hand and a five-card suit they should bid the long suit as a weak take out. With 9+ hcp and a balanced hand you should re-double, to play! However, you always found your opponents ran from the re-double and you never played 1NT re-doubled.
There are practical alternatives to this aged method. A commonly used one by many tournament players is Halmic (sometimes called Helmic). Using this method, responder can describe four types of hand! Remember that in all these situations it is assumed that opener cannot have more than one doubleton – be wary if you open 1NT with 5422 shape.
1) A weak hand and any 5-card (or more) suit, responder should re-double. Opener must bid 2♣, and responder then leaves 2♣ in or converts to the relevant 5+ card suit.
2) A two suited weak hand (4-4 equal length), responder should bid the lower of the suits. Opener should pass with a three-card suit support, or bid the next suit up (in their hand, which may only be 3 cards) with a doubleton of responder’s suit. This sometime results in playing in a 3-3 fit.
3) A weak 4-3-3-3 hand, responder should PASS. As opener you must re-double should your Right Hand Opponent (RHO) pass. Responder should bid 2♣ to show a 4-3-3-3 shape. This bid ensures a 4-3 fit at least will be found. Opener should bid his longest suit in an attempt to escape un-doubled.
4) With a hand prepared to play in 1NT doubled, responder should again PASS, forcing opener to re-double (as in case 3). Responder can then leave the re-double in and “fun” follows! However, be warned, this can be quite dangerous at TEAMS scoring and you may go down loads of points…
So summarising, after the double of 1NT :