1NT opening shows either a balanced 10-12 or a balanced 16-18. 
Responder bids 2C to ask and opener rebids:
1) 2D, 2H or 2S with 10-12 
2) 2NT with 16
3) 3C with 17-18
Responder can then bid 3D to ask for a four card major.
Other responses of 2D, 2H and 2S are signoff-- even opposite a 
strong NT opening. 

If 1NT is doubled, opener redoubles to show the strong hand.