Not just 'Tetris' (and *-'tris') trademark and Lanham Act trade dress, but they have also successfully argued in court [1] that the 'look and feel' of Tetris constitutes its copyrighted expression. In that case, Tetris Holding argued (and the judge agreed) that even the use of the seven one-sided tetromino playing pieces was part of their copyright.
Here's the description of the only Tetris gameplay mechanics the judge in this case conceded cannot be protected:
> Tetris is a puzzle game where a user manipulates pieces composed of square blocks, each made into a different geometric shape, that fall from the top of the game board to the bottom where the pieces accumulate. The user is given a new piece after the current one reaches the bottom of the available game space. While a piece is falling, the user rotates it in order to fit it in with the accumulated pieces. The object of the puzzle is to fill all spaces along a horizontal line. If that is accomplished, the line is erased, points are earned, and more of the game board is available for play. But if the pieces accumulate and reach the top of the screen, then the game is over. These then are the general, abstract ideas underlying Tetris and cannot be protected by copyright nor can expressive elements that are inseparable from them.
Disclaimer: I chose the colors from first principles and drew the graphics for Xio's version from scratch in Photoshop without any reference (as a volunteer; it was my friends' company), had to sit through depositions about this, etc.
In theory the Tetris Company is incorrect on the merits (the features they claim as "trade dress" are clear aspects of the game rules, not ornamentation, and in Xio's game had quite notable gameplay differences, especially in its multiplayer variants, and significant graphical style differences) and a discerning judge should see through the argument and find against them.
In practice judges often make a snap judgment based on personal prejudices (in this case, that the appearance amounted to "wholesale copying" that "pilfer[s] another's creativity" "without offering any originality or ingenuity of its own" and that such features as the way the pieces move falling from the top of the screen and the dimensions of the game board are "expression" rather than gameplay, and "Xio was also free to design a puzzle game using pieces of different shapes instead of using the same seven pieces used in Tetris." etc.) and ignore the letter and spirit of the law, filling in the decision starting from the conclusion. It has to date been too bothersome for someone with deep pockets to contest this to the appellate level.
I agree, the judgment read like a post hoc rationalization from someone unfamiliar with game development, uncritically parroting arguments of the side with a deeper bench.
What surprised me most from the ruling was how it took the legal doctrine of idea-expression dichotomy and twisted it into absurdity, hinging on an implicit argument that a game design essentially serves no utilitarian 'function' or 'purpose'. Core mechanics or essential gameplay rules like the tetrominoes or the 10-block width of the playfield are thus marked 'expression' or 'arbitrary flourishes', leaving a vague, unidentifiable abstraction of a falling-blocks puzzle game as the only unprotectible set of game rules that form its 'idea'.
Xio was free to design a puzzle game without tetrominoes, or downward+lateral+rotating movements, or a 10x20 playfield, and it would still 'function' just as well (that is: not at all, it's just a game). A chess game developer is equally free to design a strategy game without an 8x8 board or the knight's distinctive L-shaped movement, but it would no longer be chess, it would be rules for a completely different game.
Here's the description of the only Tetris gameplay mechanics the judge in this case conceded cannot be protected:
> Tetris is a puzzle game where a user manipulates pieces composed of square blocks, each made into a different geometric shape, that fall from the top of the game board to the bottom where the pieces accumulate. The user is given a new piece after the current one reaches the bottom of the available game space. While a piece is falling, the user rotates it in order to fit it in with the accumulated pieces. The object of the puzzle is to fill all spaces along a horizontal line. If that is accomplished, the line is erased, points are earned, and more of the game board is available for play. But if the pieces accumulate and reach the top of the screen, then the game is over. These then are the general, abstract ideas underlying Tetris and cannot be protected by copyright nor can expressive elements that are inseparable from them.
[1] Tetris Holding, LLC v. Xio Interactive, Inc. (2012) https://scholar.google.com/scholar_case?case=180648822600252...