The Lebensohl Convention

When 1NT is overcalled, a bid by the responder at 3 level is game forcing. 

If the responder can show her preferred suit at 2 level then she does so. 

If the preferred suit  cannot be bid at  2 level then the conventional bid of 2NT is used to avoid a game forcing bid. The opener must rebid the relay of 3C.

After the rebid of 3 clubs The responder can pass or bid her preferred suit at the three level. 

Stayman is still available to the responder by bidding the overcaller's minor suit (a cue bid). 

If the responder bids a major at 3 level after an intervention  then the opener can chose between game in the major or 3NT if holding 2 stoppers in opposition’s suit.