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;
The following redouble is obvious SOS:
SOS redoubles are not used after a penalty double of 1NT, unless conventional runouts are used.