Corrections and additions in the Nimzo-Indian Defense

[seccohazy]

I have consulted opening books by Gligoric (1985), Hansen (2001), Sokolov (2021), Swiercz (2021), as well as online sources and E.C.O. Volume E. All of them contradict the lichess opening data set in some respects. Surprisingly enough, the wikipedia article provides the most coherent and complete picture, I recommend using it as a reference point.

For a start I propose these changes:

• Removing lines 59 and 60 — Incorrect ECO numbers, dubious opening names. These are transpositional moves, the positions are frequently reached via the Queen’s Indian Defense and should probably inherit the name from previous moves. “Duchamp Variation” does not appear in any of my sources.
• Removing the redundant “Sämisch Variation, Accelerated”. As noted in PR Nimzo-Indian Saemisch Variation starts after 4.a3 #218, the normal move order Sämisch begins with 4. a3.
• Changing “Normal Line” and “Normal Variation” to “Rubinstein System”, as Rubinstein is by far the most common name for the 4. e3 complex and probably what people are looking for when they search for it. (Though, granted, it's not a particularly elegant solution either, given that there’s a separate “Rubinstein Variation” within this system.)
• Correcting “Hübner Variation” — Sources give 6… Bxc3+ at the earliest, and more commonly 7...d6. It doesn't makes sense to have the Rubinstein Variation subordinated to Hübner’s.
• “Fischer Variation” — Most sources give 5...Ba6. The current name for this move is “Bronstein (Byrne)”, which is almost certainly incorrect. In the same line I have added the “American Variation” and the “Romanishin–Psakhis Variation”. All three are subordinated to the "St. Petersburg Variation", which is a bit of a lichess eccentricity.

[niklasf]

Thanks!

[ljudkvalitet]

Sorry, I'm new here and my comment may not be very interesting.

Regarding 1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. Nc3 Bb4, it's also called the Duchamp Variation on Chesstempo and Chess.com. I don't know where they got that name from and maybe it's irrelevant because this is Lichess. Mr. Duchamp himself played this line in the 1933 Olympiad against Eduard Glass but that's the only game I can find where he played it and he was also not the first player to arrive at that position. Still, I feel it would be nice if the name was kept because it's a common line in practice but I'm not sure if I have a convincing argument why it should be specifically named after Duchamp.

I can't comment on the other changes but overall it sounds like you've done some solid research.

[seccohazy]

Regarding 1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. Nc3 Bb4

There's no absolute correct about these naming conventions, but to me the main issue with the variation is its highly transpositional nature — it's a perfect hybrid of Nimzo-Indian and Queen's Indian moves. Regardless of move order, the resulting position doesn't have its own ECO number. I'm not sure if Duchamp's game in the 1930s is enough of a reason to give the position a fixed "E21" Nimzo-Indian number, when the theory has largely done without a mention of Duchamp, largely developed in other directions.