The Romex system is a bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

 bidding system designed by the Mexican bridge expert George Rosenkranz

George Rosenkranz

George Rosenkranz is a Mexican scientist in steroid research and a professional bridge player. He was born in Hungary, educated in Switzerland and lived in Mexico for 66 years...

. Key features of Romex are the multiple meanings attributed to certain bids, namely the Dynamic one notrump and the Mexican two diamonds.

Dynamic one notrump

The dynamic one notrump opening indicates either:

• a balanced or unbalanced hands in the range of 19-21 high card points (HCP) and six controls, or
• an unbalanced hand just short of a 2 opening.

The most common responses to the dynamic one notrump opening are:

• 2 – negative (0-5 HCP)
• 2 – 6+ HCP, game force
  Forcing bid
  In the card game contract bridge, a forcing bid is any bid that obliges the partner to bid over an intermediate opposing pass. Owing to the partnership's bidding system or a bridge convention, partner must "keep the bidding open", i.e...
  
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Higher level responses indicate control count.

A consequence of the use of the dynamic notrump is that the 1NT rebid by opener becomes wide-range (typically 12-16 hcp). With game invitational values or better, responder reacts to such a rebid with a 2 checkback convention to ask opener to further describe his hand.

Mexican two diamonds

This opening indicates one of four hands:

• a balanced hand with 21-22 points and seven controls
• a balanced hand with 27-28 points and 10 controls
• an unbalanced game-forcing hand with diamonds as the longest suit or
• a three-suited game-force hand.

Other calls

One-level and 2 openings are as per Standard American

Standard American

Standard American is a common bidding system for the game of bridge in the United States, also widely used in the rest of the world. This system, or a slight variant, is learned first by most beginners in the U.S. and may be referred to as 'Goren'; a dominant version used in on-line computer...

with 3+ minors and 5+ majors and limited by the failure to open one notrump.

Romex employs special asking bids in lieu of cuebidding, and for slam investigation.