A bid of 2♣ in response to 1NT shows enough values to invite game, but not enough to force to game. The range for this bid is 13 (or a very good 12) to a bad 14 HCP. Opener's bids 2♥ or 2♠ with a 5 card major, 3♣ or 3♦ with a 6 card minor or bids 2♦ with any other hand. After 2♦ you bid 4 card majors up the line.
2♦ shows no 5 card major or 6 card minor
- Bid up the line to show 4 card and stop at 2NT with min, opener should jump if you know game values
2♥/♠ shows 5 of the major
3♣/♦ shows a 6 card minor