NT Invitational Response

This bid shows a point range such that when added to the top of opener's point range, the combined hands have enough points for game.

The basic invitational response to 1N is simply 2N, but when 2N is used as a transfer to Diamonds, the 2N raise is not available. In this case, responder first bids 2, which is purportedly Stayman, but after any response by opener (2, 2, or 2), responder bids 2N.

After 1N-2, 2-2N, responder may either have 4 of the other major (if he was bidding a real Stayman) or he may not have ever had a 4-card major and is just inviting to game in notrump. If opener wants to check on the other 4-card major, he can accept by bidding 3 of the major over responder's 2N, such as:
1N-2
2-2N
3
and now responder can bid 4 if he has a 4-card Heart suit or 3N. Note that opener cannot bid 3 unless he is at the top of his notrump range, since he is forcing to game.