The US District Court of New Jersey has ruled that Tetris-like game Mino infringes Tetris Holding's copyright, Sunstein Law reports (via Marsh Davies ). It's a decision that'll have a big knock-on effect for all games that are functionally similar or identical to already successful games.

Previously, developers have struggled to prove that the mechanics of a game can be protected under copyright, since 'ideas' themselves cannot. That's led to an incredibly clone-happy culture, in which virtually every successful game has any number of rip-offs on Flash portals or the iOS App Store.

But this case delved deep into the mechanics of Tetris to sort out what qualified as the 'idea' of the game, and what elements were specific to Tetris' particular 'expression' of it. Copying the idea is fine - copying the expression is not.

Mino creators Xio didn't try to argue that the similar elements of their game weren't copied from Tetris. Instead, they claimed that the specifics of Tetris are essential and generic to the idea behind it, and therefore fair game.

The court's decision, quite reasonably, was that it would be perfectly possible to make a game about falling blocks fitting together and vanishing without copying the specific shapes of those blocks, the dimensions of the game space, the brightly coloured look, and many other specifics of Tetris that Mino shares.

You can see what Xio might have been thinking: change any or all of those things, and you'd probably end up with a worst game. But that's not Tetris Holdings' problem: if you choose to copy their idea, you have to come up with an expression of it that's different to theirs. If you can't think of a better one, tough.

The result will give hope to developers like Spry Fox, who are taking legal action against the creators of Yeti Town, which they say copies their Facebook game Triple Town. There's no denying the ideas are basically the same, but Spy Fox point out that Yeti Town also copies particulars like the specific prices of items in the in-game shop. Previously, Yeti Town developer 6Waves might have been able to argue that these specifics were part of the 'idea' of the game and not the 'expression' of it. The Tetris decision is going to make that much harder.

From everything I've read about the case, it seems like the right call, and a positive thing for developers suffering actual damage from clones. But the specifics are really important. It would be a disaster if all game mechanics were declared part of the 'expression' of an idea, because games would no longer be allowed to learn from each other. Game design is as much science as art, and all of my favourite games copy good mechanics from the classics that inspired them.

If every legal case goes into the kind of analytical depth that this one did, hopefully that distinction will remain clear. Sunstein's write-up is great, by the way, and explains what little legalese is relevant.