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In an unusual decision, an appeals court in Washington ruled on Friday that Oracle(s orcl) can copyright application programming interfaces (APIs) for the Java programming language. The ruling is a defeat for Google(s goog), which uses the APIs for its Android software, but also has implications for the technology industry as a whole, where APIs — which let computer programs speak to each other — are considered to be a basic building tool and outside the scope of copyright.

In a unanimous three-judge ruling, the U.S. Court of Appeals for the Federal Circuit held that a district judge in California erred in 2012 by declaring that the APIs were simply a function or an idea, and not an expression subject to copyright protection.

The appeals court found instead that, despite the fact Google had written its own code to implement the software, it had infringed copyright by using Oracle’s “declaring code” — which represents headers and other basic directional signals — to arrange it.

The ruling is significant because it goes against traditional understandings of the “idea/expression dichotomy” under copyright law, which holds that a form or concept can’t be protected, but that a specific expression of it can be; for instance, the structure of a sonnet is not copyrightable but a specific poem is. In the case of the Java APIs, the appeals court has given Oracle a monopoly over what appears to be a functional concept.

In a remarkable passage, Judge Kathleen O’Malley compares the short names such as “java.lang.ref” and “java.lang.reflect,” which Oracle uses to name the APIs, to great works of literature:

By analogy, the opening of Charles Dickens’ A Tale of Two Cities is nothing but a string of short phrases. Yet no one could contend that this portion of Dickens’ work is unworthy of copyright protection because it can be broken into those shorter constituent components.

After concluding the APIs were copyrightable, the appeals court referred the case back to a jury to determine if Google qualifies for a “fair use” exemption.

O’Malley’s decision was met with incredulity by technology experts and intellectual property scholars, many of whom had applauded the earlier ruling of the California judge, who had taught himself Java before ruling on the Oracle-Google trial. Here are some reactions to the new decision on Twitter:

Is there any body of IP law that the Federal Circuit hasn't done its best/worst to screw up? — James Grimmelmann (@grimmelm) May 9, 2014

But oh boy the Federal Circuit are really making the Supreme Court look like a Genius Bar — Parker Higgins (@xor) May 9, 2014

The Federal Circuit has already screwed up patent law. I'll be very annoyed if they break copyright law too. — Timothy B. Lee (@binarybits) May 9, 2014

Google has yet to say if it will appeal the decision, and only provided a short statement by email:

“We’re disappointed by this ruling, which sets a damaging precedent for computer science and software development, and are considering our options.”

As the comments on Twitter suggest, the controversy may focus renewed attention on the role of the Federal Circuit, which is responsible for all patent appeals in the country but, in this case, issued a rare copyright ruling. The court has been criticized as “rogue” and has been repeatedly overturned 9-0 by the Supreme Court.

Even if Google successfully challenges the Federal Circuit’s ruling, the process could take years, and result in widespread legal uncertainty and litigation costs.