1♣-1♦ 2NT-3♦ I like to play that all bids over 2NT are forcing, so this only shows a 6-card suit. Of course, it shouldn't be done on a normal hand that will gladly play 3NT but more on a hand that has 'a problem'. Opener would then decide whether to go on with 3NT, show his best Major (if he shows spades the final contract might be 4♠) or support diamonds and try for slam or just for 5♦.