Vampyr, on 2020-March-03, 17:59, said:
Oh, I hadnt realised that the context was after a 2/1 response. But always is an overbid. I believe that when the requirements for a 2/1 were considerably weaker, 8HCP or thereabouts, a 2NT rebid was not GF. And I believe that some people still play it that way.
This is true. The original Weak NT Acol (played at the time only non-vul!) had 1X - 2Y; 2NT as invitational and non-forcing. It is a comparatively recent (early 1980s?) development to up the minimum requirements for a 2 over 1 response and make the 2NT rebid game-forcing.
In terms of the discussion on 2/1, it is worth mentioning that not even everyone playing 2/1 GF plays 1
♦ - 2
♣ as game-forcing. It is one of the auctions that one should typically ask a new partner about. For a passed hand, the auction 1
♦ - 2
♣ has a couple of specialised possibilities beyond pescetom's "shaded" definition. Some, particularly those who play 1
♦ - 3
♣ by an un passed hand as invitational, move that hand down to 2
♣ for a passed hand and keep the rest of their system essentially the same. Some others, particularly those who like to play 1M - 2
♦ as their passed hand Drury, use 1
♦ - 2
♣ from a passed hand to show the equivalent of a Weak 2 opening in clubs. I think most common though is to revert to SA a response style structure, leading to the potential rebid issue CY is referring to.