If your partner's last bid was notrump, there are three ways you can end up in slam or grand slam:
Invite 6NT. A bid of 4NT is called a quantitative raise:
|NT 4NT: Invites 6NT. (Partner should pass or bid 6NT.)|
Invite 7NT. A bid of 5NT is a grand slam invitation:
|NT 5NT: Invites 7NT. (Partner should either bid 6NT or 7NT.)|
Note that partner must not pass the 5NT: you have decided to go to slam, and are inviting grand slam!
Bid Slam. A bid of 6NT or 7NT is final. Partner should not raise a bid of 6NT to 7NT.
Example: After a 2NT Opening. For example, after a 2NT opening (showing 20-21 HCP), you would make the following bids: