The Moscow Defence

Where a 1NT trump bid is doubled the Moscow convention can be used.

After the double the responder passes which is alerted as a conventional bid. 

The opener must rebid a five card suit or, not having one, redouble. 

If the total value of the hands is less than 20 then the openers five card suit is the best option. 

If the rebid was a redouble then the responder must bid her five card suit or the best 4 card suit. 

If the total hand strength is more than 20 then the redouble can be left in with the expectation of a good score.