Good raise after overcalls
A cuebid of the opponents suit is used to show a good raise in partners major suit.
Example 1:
A J 9 8 3 Q 7 5 2
10 5 3 A J 7 6
Q 9 K 8 3
A J 3 5 2
 
1S                                  (2D overcall) 3D = Cuebid (Good raise)
3S = minimum pass
 
Example 2:
A K 9 8 7 Q J 3 2
A 5 K 3 2
6 4 A Q 3
A Q J 2 9 8 3
 
1S                                 (2H overcall) 3H = Cuebid (good raise)
4C = natural, showing extras 4D = cuebid
4NT = RKC Blackwood 5D = 1 ace
5H = queen ask? 6H = yes, and king of hearts
6S pass