An SOS redouble is used after a penalty double, to request a different suit from the partner. It applies when

  • we have not agreed the doubled suit;
  • the contract is not bid by a one-suited hand (such as bidding the contract twice)
  • there are at least 2 unbid suits;
  • the auction is at a low level;

It is commonly used after a penalty pass of a takeout double.

The following redouble is obvious SOS:

South West North East
1 / / X
/ / XX

SOS redoubles are not used after a penalty double of 1NT, unless conventional runouts are used.