Blue Book 5A5
"A partnership may play two basic systems at different positions or vulnerabilities only in Level 4 or Level 5 competitions, and only where rounds are of 7 boards or more. The partnership must display two system cards for each system, indicating the occasions when the different systems apply.
It is always permitted to vary certain parts of a system according to position and/or vulnerability. This includes, for example, variable NT openings and playing four or five card majors in different positions."
This may have come up before, but when does changing parts of a system result in a different basic system (and has some interesting philosophical connotations).
Our current system is loosly described as "Benji Acol with multi 2D and Lucas 2s" - which ipso facto is quite a deviation - however the principle is that 2 Clubs could be as weak as 8 playing tricks with the hand not strong (unless the suit is clubs) and 2 Diamonds is multi including very strong hands; and in theory 4 card majors.
So we decide that having weak multi options and Lucas in third and fourth position is probably not the optimum strategy (you don't need a pre-emptive bid in fourth and in third it is probably best that partner knows your suit since there is bound to be interference if you are relatively weak. We figure that having natural weak/ intermeidate 2s is probably the best trade-off between pre-emption and knowledge - and the meaing of 2D would correspondingly change.
So: does the first or the second paragraph apply? When does varying parts of the system result in a change of basic system or (more precisely) - what is the 'kernal' of a system?
To give an (extreme) example. If you play 4 card majors and a weak no trump in positions one and two (ACOL), and 5 card majors and a strong No Trump (a la SAYC), then those changes alone do not change the basic system according to paragraph 2.
Comments
Not just a deviation, but a contradiction!
Some people play what is effectively a Benji 2C without the corresponding 2D; they just push all hands that are either a Benji 2C or a Benji 2D through 2C. These pairs tend to either have complex followups to distinguish what sort of hand opener has, or else just rely on Benji-2D hands hardly ever coming up.
Disclosing such a system is quite awkward because it tends to defy people's expectations; they'll assume you have something else regardless of how you describe it. I think of it as "Double Benji" sometimes because you're playing Benji 2C and Reverse Benji 2C at the same time.
For what it's worth, I don't think a change to two-level options in third seat is enough to produce a different basic system. (Most people change their two-level and three-level options in fourth seat, after all.)
On a related subject, having some objective way to define whether two basic systems were the same would be very helpful to me for an entirely separate project (recording which basic systems the various players in an event are using in a way that avoids unnecessary duplication but capturing important differences). I'm currently leaning to considering the most important aspects of a system being the artificiality/naturality of the 1-level bids, the length guaranteed by the 1-level suit bids, the notrump range (although this often changes in third seat or with vulnerability, so probably shouldn't trigger 5A5), and the strength suggested by a 2-over-1 bid by responder. But it's hard to capture in an unambiguous definition!
The regulation is indeed very vague, and I've deliberately not tried to 'fix' it in the past.
The problem is that whether two systems are 'different' isn't really a function of the basic things written on the card, it's a matter of how much discussion and debate you've had with your partner.
If you are taking a partnership system seriously, then weak NT and strong NT are fundamentally different. The strength of the NT opening should affect virtually all of your competitive agreements; how heavy or how light you invite as responder if partner raises your major suit response; your responses to 1NT; how light you open with a semi-balanced hand and all sorts of other things. Similarly 5-card majors and "real" 4-card majors (always open the major) are really completely different.
However, many people who play varying NT ranges don't go into all this detail and basically play exactly the same except for switching round the NT openings and responses. Similarly, many people play 5-card majors but allow a good 4-card major in 3rd seat. That isn't really changing their system and all the rest of their agreements, but they just have the additional agreement that AKJx Qxx xxx Qxx is a 1C opening in first or second position, but a 1S opening in third.
Trying to put all of that into regulation would just confuse most people and would be far too long, so for the purpose of applying the regulation I go back to its purpose.
The reason we don't allow multiple systems against the same opponents is because of the extra time it may take them to discuss their defence. So if the 'second' system needs additional discussion by the opposing pair, then it 'counts'. This still isn't quite right, because so many people have different defences against a strong and weak 1NT opening, but that's an exception because it's so common. And there also seems to be a basic assumption that you can muck around with your pre-emptive openings without changing your system (e.g. multi in 1st and 2nd seat). So maybe I'd rephrase as does the 'second' system need a change in mindset from the opponents? If you open 1S in a strong club system, that is very different from a 'standard' 1S opening but isn't alerted and needs a change in approach from the other pair (both in the bidding and in the defence).
So to take a concrete example. I would rule that playing strict 4-card majors (bid the major first, even with a 5-card minor) in 1st and 2nd, but switching to 5-card majors combined with opening 1C on all 5332s (any suit) outside the NT range in third and fourth IS two different systems; but simply moving from 'Acol' as taught to 5-card majors better minor, isn't. This is because defending against an always-unbalanced 1D/H/S opening combined with a short club is very different from Acol (it has a big impact on whether you should double or overcall 1NT for example).
But I realise this is all a bit murky and is starting to look a bit like pornography (I know it when I see it)
Taking this sentence in isolation made me smile.