Advocates of open source software have hailed a court ruling protecting its use even though it is given away free.
The US federal appeals court move overturned a lower court decision involving free software used in model trains that a hobbyist put online.
The court has now said conditions of an agreement called the Artistic Licence were enforceable under copyright law.
"For non-lawgeeks, this won't seem important but this is huge," said Stanford Law Professor Larry Lessig.
"In non-technical terms, the Court has held that free licences set conditions on the use of copyrighted work. When you violate the condition, the licence disappears, meaning you're simply a copyright infringer.
"This is a very important victory."
Copyright infringement
According to details outlined in the ruling, Robert Jacobsen had written and then released code under an Artistic Licence. This meant anyone using that free code had to attribute the author, highlight the original source of the files and explain how the code had been modified.
Mr Jacobsen, who manages open source software group Java Model Railroad Interface, accused commercial software developer Matthew Katzer and his company of ignoring the terms of the Artistic Licence when they took his code and used it to develop commercial software products for trains.
An earlier court ruling did not agree with Mr Jacobsen's stance that Mr Katzer and his company had infringed his copyright and said the licence Mr Jacobsen used was "intentionally broad." Instead the court ruled he might be able to claim breach of contract.
Legal experts have said the distinction is important since under federal copyright law a plaintiff can seek statutory damages and can be more easily granted an injunction than under contract law.
But now the US appeals court "determined that the terms of the Artistic License are enforceable copyright conditions".
"Copyright holders who engage in open source licensing have the right to control the modification and distribution of copyrighted materials," Judge Jeffrey White wrote in his 15-page decision.
"Open source licensing has become a widely used method of creative collaboration that serves to advance the arts and sciences in a manner and at a pace few could have imagined just a few decades ago," Judge White said.
'Welcome change'
Mark Radcliffe of the Open Source Initiative said: "Although the reasoning is limited to the Artistic Licence and the interpretation of each open source licence will depend on the wording of its provisions, this decision is a welcome change."
The ruling has implications for the Creative Commons licence which offers ways for work to go into the public domain and still be protected. These licenses are widely used by academic organisations like MIT for distributing coursework, scientific groups, artists, movie makers and Wikipedia among others.
Creative Commons filed an amicus or friends brief on behalf of Mr Jacobsen. Its general counsel Diane Peters told BBC News "The federal court recognised that even though licensors give up some rights it doesn't mean they have any less rights to access the remedies our law provides.
"This opinion demonstrates a strong understanding of a basic economic principle of the internet; that even though money doesn't change hands, attribution is a valuable economic right in the information economy."
If the case had gone the other way it would have been a real blow to the open source movement, according to Jeff Neuberger a partner at Proskauer Rose LLP.
He told the Wall Street Journal: "Lots of companies rely on open source, and if they had lost their ability to enforce their rights they would have shied away from the software."
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