Adopting Colorado 2clubs updates how you can play Extensile 2 clubs over 1 of a major
When your partnership adopts the Colorado2Club conventions you have a great bid to show those awkward 18-19 HCP hands.
In the Extensile 2clubs convention, we have a bid sequences after 1 of a major - 2♣ to show a balanced 18-19 HCP. We no longer need this bid because we would have opened an 18-19 HCP with a 5-card major as 2♣ instead of one of the major. How does this change our Extensile 2♣ bidding?
Let's examine both Colorado 2♣ and Extensile 2♣ in more detail.
For Colordo2♣, a balanced hand with 18-19 HCP is opened as 2 clubs. This includes choosing to open with 2 clubs when holding a 5 card major. We have bidding tools to uncover hand that opened with a 5 card major. The exception, is when we hold 5 card in on major AND ONLY 2 cards in the other major. The reasoning for avoiding hands with 5-2 major suit shape is the possibility that partner will transfer us to our 2 card suit, when they hold 5 cards. We end up playing in a 5-2 trump fit. If responder also hold 3 cards in the other major, we just missed an 8 card, 5-3 fit. This is most likely when responder is weak and is planning on passing after the transfer bid. There is no room to figure out this 5-3 fit. Also, when responder is too weak to consider 18-19 HCP encouraging to bid again, we would really like to be playing for an auction at the 1-level. Therefore, with those 5-2 major suit shaped hands, we prefer to open 1 of a major instead of 2 clubs.
Looking at the Extensile 2♣. The bid of 2♣ over opener's major suit opening promises a couple of types of hands. The most likely are either a limit raise in opener's suit or an invitational strength hand without support of opener's suit. Two less frequent hand are also possible, responder may have game forcing values and a club suit (5 or more cards) without support for opener's suit. This bid is choosen instead of the game forcing 2♦ because it is easier to explore bidding room and makes a less complicated auction. Very rarely, responder might have a slightly less than invitational strength hand, 6-card club suit and also no support for their partner. They are angling the final contract to a 3 clubs part score.
Responder is expecting the opener to make the relay bid of 2♦, and then they show which of the four types of hands they hold. The opener is not restricted to only bidding 2♦, they can break of the relay with unusual/expceptional hands. Opener can break the relay request and bid 2NT when they have that balanced hand between opening 1NT and 2NT; 18-19 HCP. There is some wiggle room for opener to upgrade an 17 HCP into this range - this might a hand that chose not to open 1NT because of a weak 2 card doubleton in the other major.
We can see that now playing both conventions, the only possible hand that bids 2NT - breaking the relay - is a flawed balanced hand in that point range. That may be fine. Breaking the relay is a extreme bid by the opener.
However, remember responder has promised an invitational hand. So, if opener has 18 or 19 HCP, they could leap directory to 3NT. At this point partner can pass or correct (if they had a limit raise type of hand). If they had a game forcing hand with clubs, they can start cue bidding at the 4-level to start slam investigation. And, if they were a middleton hand with 6 clubs, 3NT is still a very viable contract.
The wart in the situation is that responder knows that opener has a 5-2 shape int he majors and if they end up playing in no-trump AND responder is also short in that major, opponents might run off 5 tricks to set the contract. Responder should have some latitude to evaulate their major side suit for a potential stopper. Remember, opener could hold a Qx or JT doubleton and responder's partial stop promotes to a solid stop. Overall, responder should only scramble with a weak doubleton, or singeton. And, if that is the case, why aren't the opponent's getting into the auction with a 10-card major suit?
However, we have other hands that have a 5 card major, 2 card side major with 15-17 HCP that won't open 1NT in favor of opening 1 of the major. If we update our meaning of breaking the relay by bidding 2NT to show just 15-17 HCP, we haven't lost anything. 2NT shows a flawed balanced hand with 15-17 HCP. Typically, responder has ~11 HCP. That means game. Responder can now: bid 3NT with invitational hand and no support, bid 4 of the major with limit raise type hand. They can decide to pass the 2NT or bid 3C when they started with the weaker 8-10 HCP and 6 clubs or raise to 3NT if they have good clubs or a solid side suit entry to run them. When responder has game force values (13+ HCP) they know an upper limit (of 17 HCP) in opener's hand. They would typically need 15+ HCP to begin to consider slam investigation. Any other 3-level bid (or 4C) should indicate they started with game force in clubs type hand and think slam is possible.