Unbalanced Diamond systems or versions of....

Here's a long comment, split into five sections. The sections are:

Constructive options over 1♣.
Constructive options over 1♦.
Competitive options.
What Chip Martel's CC says.
What I prefer.

I'll focus on the differences between 'unbalanced diamond, balanced club' compared to standard, rather than explaining the system from the ground up. This saves me a bunch of writing, at a possible cost of clarity. I'd like to hear what other people think about this and what I've missed.

1. Constructive options over 1♣.
In a "clubs or balanced" 1♣ opening, also known as 'balanced club' (but note that it also contains unbalanced primary clubs hands!), there are basically two mainstream systems for getting most out of the bidding space below 1NT: Walsh and T-Walsh. I am a big fan of Dutch Doubleton as a third (superior) alternative, but that system is far less widespread globally.
Walsh and T-Walsh both introduce something that I find very valuable: opener 'bypassing' major suits to show a balanced hand. In Walsh there's some room for debate on 1♣-1♥; 1NT, but at least on 1♣-1♦; 1NT there's no need to show any major suits as partner has denied holding a less-than-game-forcing hand with a 4cM. This reinforces the NT ladder and clarifies the 'clubs or balanced' question which we took on with the balanced 1♣. T-Walsh goes a step further, but first a little aside. There are a lot (a LOT a LOT) of different T-Walsh versions - I've previously joked that there are more T-Walsh versions than there are partnerships playing T-Walsh, since everybody invents a new one and some are lost to time. I can't and won't cover them all, but there are two relatively mainstream options, split by how opener responds to a transfer:

Opener's transfer completion shows exactly 3-card support, other bids showing either 2(-) or 4(+). Typically a transfer completion is 99% forcing, limited only by failure to open a strong 2♣ and possibly excepting some special jump bids. With this approach a transfer rejection with 1NT typically shows a minimum balanced hand with exactly a doubleton in support.
Opener's transfer completion shows a weak NT hand with at most 3-card support, or maybe some difficult hands that have no better bid. A transfer completion is not forcing as the bid is limited (in a strong NT context, typically to about 14 hcp). With this approach a transfer rejection with 1NT typically shows the hand too strong for the 1NT opening with at most 3-card support.

I strongly prefer the second type, and think it goes really well with balanced club. I'll focus on that for now.
In such a T-Walsh structure opener can clarify their balanced hands cheaply at the 1-level. The NT ladder becomes really efficient: 12-14 open 1♣ and complete the transfer (unless partner hit your 4cM, then you jump complete the transfer), 15-17 open 1NT, 18-19 open 1♣ and rebid 1NT (unless partner hit our 4cM - now we need exceptions). All other rebids show an unbalanced hand without support. It's so easy and at a low level, while clarifying degree of fit, hand type (balanced versus unbalanced primary clubs) and strength. I've previously mentioned my 'double shift' - I hope it's no surprise that with a system like this and a hand like 3=3=4=3 you'd prefer to open 1♣ to 1♦ with the expectation of showing so much about your hand at the 1-level!
T-Walsh also helps resolve one other weakness induced by the balanced club: the risk of 1♣-all pass. T-Walsh allows for light responses to a 1♣ opening, secure that opener won't jump with the big balanced hand. There is still some risk of responding light - other jumps do exist - but it is significantly reduced compared to standard systems. This means we will regularly have an alternative to 'pass' available, even on weak hands, and we can reduce the risk of playing in some poor 2-3 fit or the likes.

The system I like, Dutch Doubleton, has a different approach but solves some of the same problems. It introduces 1♣-1♦ as a multi-way bid including many weak hands ('0-6 any distribution', though some hands in that range have alternatives with sufficient shape). Conversely, 1♣-1M is natural (4+ cards) but also positive (7+ hcp). This allows for a game forcing jump rebid 1♣-1M; 2NT, showing 18-19 balanced with or without 4-card support - 18+7 = game. Conversely, on 1♣-1♦; ?, Dutch Doubleton reserves the 1NT rebid for the too-strong-to-open-1NT balanced hand, while the weaker balanced hands go through an artificial 1♥. Just like T-Walsh, responder need not pass 1♣ unless they have an actual desire to play there (though the hands that pass and bid are different, as are the slight risks and gains), and we can show the balanced hands at the 1-level.

2. Constructive options over 1♦.
There are a few ways the unbalanced diamond differs from a standard 1♦. I've previously spilled some ink on '5543 systems', where we treat the 1♦ opening as a 4-card suit even if it can be on 4=4=3=2. Consequently, such an opening can be raised on 4-card support.
The unbalanced diamond in a 'balanced club' system is "5+ or some 4441's" in a natural 2/1 approach. I've played other versions (notably, including x=y=4=5 11-15 'too weak to reverse') and was unhappy with the results, and this approach is both most straightforward and most homogeneous, preparing for competition. A quick observation: the 1♦ opening showing these hands isn't so much a design choice as it's a consequence of putting balanced hands in 1♣. Now if we open 1♦ we automatically have an unbalanced hand with primary diamonds, which immediately implies a 5(+)-card suit or some awkward 4441-type hands - there's no other options. For what it's worth, regarding the 4441-type hands I recommend opening 1♦ only with 4=4=4=1 (opening 1♣ on a singleton is risky and possibly against local regulation) or 1=4=4=4 (rebid 2♣, awkward but it's even worse if it goes 1♣-1♠, or a transfer into spades). You could decide to split that last hand type by strength, planning to treat it was a balanced hand in the right range - I'm not a fan of this.
Getting back on track, this 1♦ opening shows 5(+) cards a good amount of the time. A quick simulation with my conditions gives a 4441-type hand approximately 10% of the time, 5 cards 50%, 6 cards 34%, 7+ cards 6%. Consequently, it makes sense to start raising this opening with 3-card support. I find that this resolves some of the more awkward aspects of constructive bidding after a 1♦ opening, without making huge changes to the responsive structure. Here is an example, and the one I recommend:

Pass: any (sufficiently) weak hand.
1♥: 4+ hearts, forcing
1♠: 4+ spades, forcing
1NT: 6-11, natural, no 4cM, semiforcing. Note: promises at least 7 cards in the minors, so opener can rebid 2♣ on a three-card suit.
2♣: 5+ clubs(!), forcing to game
2♦: [4+ diamonds or 3=3=3=4], invitational+
2♥: Whatever you prefer, I like 'approximately 4-8, 6(+) hearts'
2♠: Whatever you prefer, I like 'approximately 4-8, 6(+) spades'
2NT: 4+ diamonds, approximately 0-5 ('garbage raise, drop dead with your decent 17-count')
3♣: 6+ clubs, approximately 9-11 ('if you have slight extras with club tolerance we have game')
3♦: 4+ diamonds, approximately 6-9 ('we might have game opposite a decent 17-count')

This combines occasionally raising with 3-card support with a semiforcing 1NT, resolving some complications of natural bidding and getting a game forcing natural 1♦-2♣ 'for free'.
There are a few other ways to deal with the awkward no-major 10-11 invitational (10-11 point, give or take) hands, but here are a few thoughts:

You could put the hands with at least 3-card diamond support in 1♦-2♦. Like I said, raising on a 3-card suit is fine in such a system.
Naturally we'll put hands with a four card major in 1♦-1M. No need to make this complicated.
I've also put the hands with 6+ clubs in 1♦-3♣, splitting off those awkward hands.
What remains is... exactly 3=3=2=5. All other hand types are accounted for.

You could put this 10-11 count 3=3=2=5 in 1♦-2NT, giving opener a great deal of information and ability to place the contract. However, I'm not a fan. It's a waste of an otherwise useful bid, and the frequency with which opener is stuck on 1♦-1NT is low (it only really costs if opener has, say, a 14-15 count (34)=5=1, and our prospects in 3NT with such a minor suit misfit and having just wrongsided the contract likely aren't that great).
However, the main message is: between the option of a (semiforcing) 1NT, an inverted minor raise on only 3 cards, and 1♦-2NT available should you want it, there is a lot more flexibility compared to standard systems to handle the no-major hands.

The second big difference in constructive bidding, the most flashy one, comes with opener's rebids. This is most visible on 1♦-1M (which is also the most frequent), but also shows on e.g. 1♦-2♣ or 1♦-2♦. Since the opening denies holding a balanced hand, the NT rebids are idle. This frees them up for artificial continuations. I'll just list a few, since this story is already becoming far too long, without too much detail:

1♦-1M: transfer continuations starting with 1NT (so 1NT shows clubs, 2♣ shows diamonds etc.). This allows opener to show shape with a cheap forcing bid without going past the safety level, and lets us compress the strong and the weak hands into a single bid. This frees up jump bids to show specific fit-showing or awkward hand types.
1♦-1M: Gazzilli 1NT (my personal favourite, though it's a significant memory load). Using 1NT as "16+ any with at most 3-card support or 11-15 6(+)♦ with at most 2-card support" we can resolve strength and shape below 2♦, establish a game force at the 2-level, and show degree of fit. It also frees up all jump bids for fit-showing hands.
1♦-2♣; 2NT and 1♦-2♦; 2NT can show 'both majors', i.e. 4=4=4=1 or 4=4=5=0. These hand types are normally difficult to show, but what hand on this start is more suitable for bidding 2NT than one with both majors covered?

3. Competitive options.
Simply put: 1♣ is a weakness in competition. The frequency with which it is 2 or 3 cards is increased, so it's not completely safe to raise with 5-card support (although I do), and even if opener has 3-card support you may not want to compete in clubs with only an 8-card fit and a balanced hand opposite (so little/no ruffing potential on the side with the short trumps). Thankfully the odds that opener has a balanced hand with relatively short clubs drops significantly if the opponents compete, as partner is more likely to hold fewer cards in their long suit, but the odds still aren't amazing.
When playing such a balanced 1♣ you will miss some good partscores in both clubs and diamonds, both in and out of competition. There are gadgets to regain some equity here, notably Scrambling 2NT, but it's not a cure-all.
In addition, it takes time and experience to figure out which hands by opener are suitable for a takeout double in competition. 1♣-(1♠)-P-(P); ? and what does double show - unbalanced primary clubs with short spades? A general desire to compete? Does it require 4 hearts? These questions are not that difficult to answer, but they come up in a lot of slightly different situations. My personal thoughts are that if you reserve second round action only for the unbalanced hands you are leaving too many IMPs (or MPs) on the table, and you should be able to compete further with suitable balanced hands. This also shows one aspect of why such a balanced club, unbalanced diamond goes well with a strong NT opening. In weak NT systems it is somewhat common to reserve competitive actions for the strong NT type hand. With an overloaded 1♣ opening this puts more, in my opinion too much, pressure on double in competition.

Conversely, 1♦ is a strength in competition. Partner can, and should, raise freely with 3-card support (too bad if you find a 4441 opposite). There are situations where you'd even raise on 3-card support ahead of showing a 4 card major, though this is rare. Keep in mind that opener's hand evaluation will frequently depend strongly on your degree of fit, so raising can help partner make the right decisions. You can also put on the pressure by making preemptive raises with 4-card support. Lastly I find that sometimes the information the opponents' interference gives you is enough to get a near-complete count of the hand on the auction - there's only so many short and long suits partner can have.

4. What Chip Martel's CC says.
I find his CC to be quite easy to read. First check the 'System Summary' (right side of the first page): 1♣ clubs or balanced, 1♦ unbalanced, 1NT 14-16. Scroll down a little: transfer responses to 1♣. Scroll down a little further still: transfer rebids by opener after 1♦-1M. If you find this information to be difficult to find in the middle of all the other text, check the second page, the columns after the 1♣ and 1♦ openings, where it is repeated for your convenience. There are a few details that are not mentioned - e.g. do they also open 1♣ with 5♦332, or 5♥332(!), and which hands in their 1♦ opening contain exactly 4 cards - but I think this is clearly an unbalanced diamond, balanced club system with T-Walsh and transfers by opener on the second round after 1♦-1M. I notice that there's also transfers on 1♥-1♠, which I presume has to do with their Flannery 2♥ opening (e.g. if 1♥-1♠ shows five, only 2=5=3=3 is stuck for a rebid).

5. What I prefer.
As mentioned, I like Dutch Doubleton. I also prefer a 14-16 NT, so that all the ranges shift just a little. I believe that opening 1NT on 14-16 is a winner in a vacuum, and that this is amplified by the ability to show the 17-19 balanced hand at the 1-level. Many strong pairs already open their 1NT light, or frequently upgrade 14-counts. This choice does shift the ranges in DD a little - my 1♣-1♦ contains 0-7 almost any, not 0-6 almost any. If you're interested in Dutch Doubleton, I wrote about it in more detail a few years back, with some additional information hidden in this thread. Do note that this writeup of DD contains a mistake: I play 1♣-1♦; 1♥-1♠; 2♥ as a club-hearts reverse with at most 3 spades, the linked thread erroneously states that it shows 5♥6♣.
I've already told you my response structure to the 1♦ opening, though I've left out a bunch of the details (e.g. I prefer for 1♦-2♦ to be forcing to 3♦, not 2NT). I prefer Gazzilli to second round transfers by opener, but I will readily admit that it's more complicated with only minor gains. Most of the profits are on competitive auctions and responder's ability to evaluate their hand opposite a known long suit. Many of these preferences are small tweaks to a larger system, and plenty of 80/20 rules apply - it's much more important to be comfortable with the frequent sequences than to fix all the details.

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