THE CROWHURST CONVENTION
This convention allows you to rebid 1 no-trump with 12-16 points instead of the usual 15-16 points. This is very useful if either :
1. you play 5-card majors. For example, you have 12 points and 5 hearts and open 1H. Your partner responds 1S but you have only 2 spades and no 4-card minor. Without Crowhurst you are stuck, and have to bid 2H which should be a 6-card suit (as you have already shown five). By using Crowhurst, you can rebid 1NT.
2. you do not play 5-card majors but you know that if you open 1NT with 12/14 points and a doubleton major, partner always has five in that suit and takes you out into that suit or transfers. With this system partner can transfer knowing that you are unlikely to have a doubleton major.
You can avoid these problems by using Crowhurst and rebidding 1NT to show 12-16 points. If your partner has 9 or more points, he/she needs to find out which end of the range you are as game may be possible. To find more information, he/she bids 2C. The opener can now give a more detailed description of their hand (on which they have rebid 1NT) by making one of these bids in response to 2C :
a) 2D = “I have only 12/13 points and therefore cannot tell you any more.”
b) 2H or 2S = “I have 14 or 15 points, and if I have now bid a major that you bid at the 1-level, I am showing 3-card support. If I bid the other major, this says also that I have 14/15 points but I am denying 3 card support for your major.” (Opener, of course, cannot have 4-card support for responder’s major as the rebid would have been in support of that major rather than 1NT).
c) 2NT = “16 points exactly, and if you had bid a major at the 1-level, I have less than 3-card support, because………….”
d) 3H or 3S = “I have 16 points but I do have 3-card support for this major that you bid at the 1-level”
Opener can afford to jump to 3H or 3S with 16 points because partner has shown at least 9 points by making the 2C bid. There is also a useful negative inference in that if responder just rebids their own suit, or raises opener’s suit one level, they are denying having 9 or more points because they did not explore with the 2C bid.