Question: when do you use it?
Answer: When both you and opponents have bid and it's a competitive situation. You and partner have a trump fit.
In a nutshell: In a competitive situation, a bid of the same suit as the opponent's suit (a cuebid) says "Partner, I have 10+ points and support in your suit".
Why is it important? Because having a cuebid limit raise allows you to use 3 level or higher raises of your partner's suit as preemptive bids following LOTT. This greatly increases the effectiveness of your competitive bidding.
Important note: you could just use the cuebid limit raise with major suits only. Discuss this with your partner. See section "Why is the cuebid important?"
You can just skip most of the sections and go straight to the specification onwards if you like. I suggest, however, you will benefit from reading this article in its entirety.
Please remember to enter the survey, if you have read this article.
Normally, you would not bid the same suit as the opponent. Therefore, there is a space for you to use a bid of the same suit as the opponent to artificially signal something to your partner. For a cuebid limit raise, we are using this to signal "Partner, I have 10+ points and support in your suit. ".
Your partner in North has bid 1S. East has overcalled 2C. Your 3C bid is a cuebid.
Some examples will show the type of situation where a cuebid limit raise will be useful.
In these 2 situations, what will you bid? For hand A2, you know from LOTT that this situation is ideal for bidding 3H. You have 4 hearts plus your partner's 5 = 9, and you have only 4HCP + 2 distribution points, which would normally not justify a raise to 3H.
If you had hand A1, how will you show your invitational strength to your partner? Bid 3H? How is the partner going to tell which is which?
This is a similar problem with Hands A1/A2. Here the opponent has bid 1H, and your partner has overcalled 1S.
You have 2 situations, an invitational hand (B1) and a weak hand (B2). What would a raise to 3S mean?
Please note that in this case, your partner's 1S overcall may mean 10+ points (or whatever is in your bridge agreement). In case A, your partner is showing a normal opening hand.
In hand A1, you show your strength by cuebidding East. In Hand A2, you bid 3H to try to preempt the opponents.
In hand B1, you show your strength by cuebidding West. In hand B2, you try to preempt the opponent by jumping to 3S.
In this case, you are most of the time, asking your partner to play in NT if possible. If your partner lacks a stopper in the opponent's suit, your minor suit is a back up plan (see "Specification" for full details).
When you cuebid with a minor suit fit you are saying (slight modification) : "Partner, I have 10+ points and support for your suit, I don't have a stopper"
With Hand C1, Your partner is now on alert to try to bid game if possible. With Hand C2, your partner knows you are just pre-empting.
Same with this. With Hand D1, your partner should be on the alert to try to bid NT if possible or fallback to clubs. With Hand D2, your partner knows you are pre-emptive.
And now we arrive at the specification. It is possible that you and your partner agree to do this with only the major suit and ignore minor suits. Why that is so is discussed in the section "Why is the cuebid important?"
When your partner opens the bidding, and the opponent overcalls in a suit. Or when your opponent opens the bidding and your partner overcalls in a suit. Your cuebid of the opponent's suit is constructive and means
Partner, I have support for your suit (3+) and 10+ points (for major suits)
Partner, I have support for your suit (3+) and 10+ points. I have no stopper (for minor suits)
Although I will spell out the scenarios below, everything actually flows from the above statements in bold. When the fit is in a minor, you are usually looking for a NT contract. Note that if you had a stopper, you would be looking to bid 2NT or 3NT yourself - no cuebid is necessary.
In the bidding sequences below, opponent's bids are in parenthesis. For example, (1S) means an opponent bid 1S.
Your cuebid is inviting partner to bid game with about 13HCP+1 or 2 distribution points. Once your partner sees your cuebid, he will bid 3 or 4 level.
e.g. 1S-(2C)-3C-P.
Partner should bid
3S with a minimum and
4S with a bit more
Note that your partner may overcall with fewer points. Nevertheless, your cuebid is invitational.
Your partner will rebid cheapest 2 level bid with a minimum, 3 level bid invitational, and 4 level game.
e.g. (1H)-1S-P-2H
Partner can now show his strength constructively. He bids
2S - with a minimum overcall strength. No prospect of game.
3S - with around 12-14 pts (this is invitational. you may bid game yourself if you are slightly stronger)
4S - with 15-17 pts (note that with 18+ pts, you would be doubling and rebidding - this topic is out of scope)
e.g. 1D-(1S)-2S
Your partner will assess if a NT bid is possible. Possible bids are:
3D - no stopper and no interest in game.
4D - long diamond suit (6+), looking for possibility of 5D (around 17-18 pts)
2NT - stopper in spades, minimum hand (11-13HCP or bad 14)
3NT - stopper in spades, can bid game (good 14 or 15+HCP)
e.g. (1S)-2C-P-2S
Your partner will assess if a NT bid is possible.
3C - no stopper and no interest in game.
4C - a really long suit in clubs (6+) and inviting to 5C (around 17-18 pts)
2NT - stopper in spades, around 11-13HCP or bad 14. Invite to 3NT.
3NT - stopper in spades, good 14 or 15+ HCP
The cuebid takes care of any hand with 10+ pts. Any raise is now considered <10 pts and possibly pre-emptive!
if you have 8-9 pts, you can just bid 2 level normally. If you think the partnership has 9 cds in the suit, be prepared to raise further to 3 level. With 10 cds, raise to 4 level immediately!
if you have 4-7 pts and the partnership has 9 cds in the suit, jump to 3 level immediately (pre-emptive). With 10cds raise to 4 level immediately.
With a major suit, you may assume your partner has 5 cds in that suit. So you need 4+ of your own to make 9+,
With a minor suit, this gets tricky. You and your partner can agree to apply this only for diamond suit, both minor suits, or not at all. Either way, you make a pre-emptive jump to 3 level with 5cds in the suit (by assuming 4 cds in your partner's suit)
if you and partner play short club SAYC, you may only want to do this for diamonds.
if you and partner play better minor SAYC, you can do this for both minors if you wish.
I recommend that your partnership switch to better minor SAYC, and have pre-emptive minor responses with 5+ cds.