Vienna NT in defence is used after opening
on the 1-level. You bid 1NT that shows:
17+ Hcp and Unbalance / 19+ Hcp and balanced hand.
The responses to the NT are
2C : 0-7 Hcp.
2D : 8-12 Hcp. Game forcing.
2H : 8 Hcp +. 5+heart suit.
2S : 8 Hcp +. 5+Spade suit.
2NT: 13 Hcp +
3C : 8 Hcp +
3D : 8 Hcp +
3H : 5-7 Hcp. 5+heart suit.
3S : 5-7 Hcp. 5+Spades suit.
3NT: 8-12 Hcp. At least 5-5 in minors.
4H : Not to many honors, but very long suit.
4S : Not to many honors, but very long suit.
1NT - 2C
2D: Ask partner to bid his best major.
With 6/7 Hcp. he can jump to the 3-level.
1NT - 2NT
You may 3 club as Baron. This is
a conventions where you bid you 4 card
suit as low as possible.