Benji with a Multi and 5CM
A pair play Benji with a weak NT, 5 card majors and a multi 2C. The 2C bid is the same as in "normal" Benji but includes a weak 2 in diamonds. So 2C is either weak 2 in diamonds, 8PT in a major/9 in a minor or 23-24 balanced. Responder bids 2D with a negative and anything else with a positive. The 2D opener is game forcing as per standard Benji except that the balanced option is 25 plus.
Two questions:
(a) Is this a legal system? (Assuming that the system card is properly filled out and that the 2C and 2D bids are properly alerted)
(b) Is it wrong to describe the system as "Modified Benji" or possibly "Benji with 5CM and Multi".
Comments
a) Assuming level 4, the only potential illegality is related to "strong" options with the 2C opening that aren't actually as strong as they should be. The "9 playing tricks in a minor" option needs to either be classified as genuinely strong, in which case it requires a certain minimum amount of defence (requiring 16+ HCP is enough to ensure this); or else as somewhere between a strong bid and a pre-empt, in which case it must exclude clubs. (You can also play a combination of these two meanings, but that rapidly gets very hard to explain at the table!)
Most Benji openers haven't discussed which of these two options they want for their Benji opening; if you want to stay within the rules, you need to decide how it works (and thus how to explain it), because the opponent's defence to it may depend on which option you pick. Generally speaking, you should use phrasing like "8 playing tricks but might not be strong" if the bid is potentially lacking in defence.
b) I don't think there are rules covering how to describe systems as a whole (as opposed to individual bids). However, "Benji" normally refers to a particular classification of 2C and 2D bids, and is otherwise just Acol. I'd personally explain the system as "5-card majors with 2C artificial, 2D game-forcing". The word "multi" probably isn't the best to use when describing it: legally speaking your bid is a type of multi, but most people will understand "multi" without qualification as referring to a 2D bid that shows either a weak two in a major or a strong hand.
The Multi 2!c is becoming quite popular these days.
I would call it "Benji except the 2 !c opening is Multi" and as an afterthought add "with five card majors and weak NT" (or whatever NT).
Barrie Partridge - CTD for Bridge Club Live
The 2 Club opener can be ', 23-24 points balanced(ish), weak two in diamonds, 8 playing tricks in an unspecified major or 9 playing tricks in an unspecified minor: if the suit isn't clubs then it may not be strong".
I would not use 'Benji' unless 2C is stronger than one bids and weaker than 2D. If I can help it, I would not use 'Multi' (without immediate qualification) unless it refers to 2D which may be weak in a major and has strong options. I don't think 'Modified Benji' is useful disclosure.
"five-card majors, weak no trumps, two diamonds is our strongest bid, two clubs is multi-way"
I agree with others. If the aim of the system description is to tell opponents what you are playing then saying your system has a multi is a fail and then opponents finding out the multi is 2C and only one weak two is included not only might be a surprise but you would defend against it differently assuming you knew.