About Modern Majors
In Modern American bidding, we follow Standard American and 2/1 bidding best practices. We open hands with 12-21 HCP and 5+ cards in a major, with 1 of that major.
It is in the responses to opening 1 of a major that Modern American shines as an improvement to Standard American and 2/1 bidding systems.
The weakness in 2/1 is the garbage bin bid of the Forcing 1NT. This bid collects all hands with 6-11 HCP that may or may not support the opening major suit bid. No wonder so many auctions go astray starting with that sequence. For Modern American, we prefer the use of the 1NT response to the opening major to match that of Standard American.
Responding 1NT means the response has 6-9 HCP (tighter point range) and denies having 3-card support for the major. This allows the opener to make much better, sound decisions with their rebid (or pass).
Standard American's weakness is the lack of a game-forcing bid at a low level. In Modern Majors, we will assign the 2♦ response as an artificial, game-forcing bid. The responder may or may not be holding diamonds; the 2♦ response shows 13+ HCP. It might or might not have support for the opening suit. There are a couple of exceptions, when a hand with Game Force values may bid something other than 2♦ as first response.
We still have a need to show hands stronger than 1NT would show (6-9HCP) and too weak to be game forcing (<13 HCP). To that end, we assign the response of 2♣ to showing those hands. 2♣ shows 11-12 HCP and may or may not have support for the opener. This is called the Extensile 2♣. It is very useful to cover these hand patterns/strength. We call the bid Extensile because we will stretch the bid meaning. We will also use the response of 2♣ when we have a game-forcing hand AND a strong club suit. We use the 2♣ bid to preserve bidding room that bidding 2♦ would limit our bidding room.
Modern American Majors have more bidding improvements to respond to the opening major suit bid. You can read more elsewhere on the website. See the Category Majors to read about Compressed Bergen, Splinter Bids, Unbalanced Limit Raise, and Balanced Game Raise conventions.