After 1NT, with a very strong hand, 19+
Looking for a Slam. . ..
Balanced
If you are balanced (and you already know your partner is), then NT Slam could be on. Now it's all about brute strength.
So, do you have enough between you for a Small Slam ? (33+ HCP). Do you have enough for a Grand Slam ? (37+ points).
Invite (4NT) or assert Small Slam (6NT),
- 19-20 HCP: 4NT
- 21-22 HCP: 6NT
OR invite (5NT) or assert Grand Slam (7NT)
- 23-24 HCP: 5NT
- 25+ HCP: 7NT
(Similar calculations apply when the opening bidder switches to NT on his first rebid - it's just that he's got a defined number of additional HCPs).
Unbalanced
If you are not balanced, a suit slam is very likely. Therefore describe your hand with a jump bid to one less than game (3 of the suit). Bidding to the 4 level in a Major is weaker, effectively saying "OK, just game", see unbalanced response to 1NT