A convention, named after the English player Eric Crowhurst, whereby a bid of 2♣ by responder, after a wide-range 1NT rebid by opener, is an enquiry. For example, if the 1NT rebid by opener shows 12-16 points then the continuations are as below:
2♦ 12-14 points without five hearts or three spades.
2♥ 12-14 points with five hearts.
2♠ 12-14 points with three spades, but not five hearts.
2NT 15-16 points.