Posted 2014-January-15, 20:49
My take, in general, as to "defining characteristics":
1. "Standard American": 5 card majors, strong NT, "strong" (but they often don't really know what that means) 2♣, weak twos in the other three suits (occasionally a different meaning for 2♦, usually Flannery), 2/1 response promises a rebid. Jump shift responses may be strong or may be weak (one or the other, not two-way). Transfer responses to 1NT
2. "2/1": 5 card majors, strong NT, "strong" (but they often don't really know what that means) 2♣, weak twos in the other three suits (occasionally - more often than in SA - a different meaning for 2♦), 2/1 response either GF or GF unless the suit is rebid. Variant: In 1♦-2♣, the response is not GF, but does promise a rebid. Jump shift responses usually weak.
3. "Precision": 5 card majors, weak or intermediate NT (but some play it as strong, 1♣ is F1, 2♣ is natural, NF. Openings other than 1♣ (up to 2♦) are limited. 2♦ art, 3 suited.
4. "Acol": 4 card majors, weak NT, potentially weak 2/1, 2♣ strong and GF, other twos strong, but slightly weaker than 2♣.
5. "Romex": 5 cards majors, 2/1 GF unless the suit is rebid, artificial and F 1NT, 2♣ artificial and GF, 2♦ artificial, GF if opener has diamonds, weak twos in the majors, Suit openings at the one level are limited by the 1NT opening to roughly 18 HCP.
If what you play fits one of these, you're playing that system, even if you have variants elsewhere. You can play inverted minors or not, it's still SA or 2/1 or whatever. However, if you tack on RCO twos or some such in place of weak twos, you're not playing one of these systems, you're playing something else.
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