Mexican 2 -Diamonds
An opening bid of two diamonds to shw a balanced hand of 19-21 points. Invented by
Rosenkranz of Mexico City. The 2-D bid is part of Rosenkranz Romex system, as are the responses.
The Mexican 2d opening bid was a part of the original ROmex system, in
which an opening bid of 1nt artificially showed an unbalanced 18-21
(i trick short of a 2c opener, basically) and 2d showed a balanced 19-21
(to complete the limitation of opening 1 bids to roughly 18 hcps).
In the current version of Romex, the 19-20 point balanced hands are
handled through the artificial 1nt opener (which no longer guarentees
an unbalanced hand) and the 2d bid has gone through several different
adjustments.
The responses to the older 2d (=19-21 flat) also went through some
modification. At first, responses were transfer/puppets. 2h
meant signoff in spades or invitational in NT, 2s meant signoff in
NT or GF, and bids of 2nt+ were slam try transfers to the next higher
suit. Then, responses showed points (2h=0-4, 2s=5-10, 2nt=11+).
Source: Root & Pavlicek's Modern Bridge Conventions; Henry Sun