1NT Opener shows 10-12 HCP and denies a 5 card major. Responder's bids, including 3 level bids and 4, are almost all to play. There are four exceptions:

Stayman

The Stayman responses are slightly different from standard: After Opener's Stayman response, suit bids show 5 card suits. 2 level bids are invitational and not forcing, 3 level bids are game forcing. 4 is Gerber, ace asking.

Interference

When the opponents interfere with 2, then double shows Stayman; over other 2 level bids, we use Lebensohl and double for penalty.

Direct Seat Doubles

Responder has various options, mostly runouts.

Balancing Seat Doubles

Opener gets first say: