In a crucial win for the free software movement, a federal appeals court has ruled that even software developers who give away the programming code for their works can sue for copyright infringement if someone misappropriates that material.

The decision by the U.S. Court of Appeals for the Federal Circuit in Washington, D.C., helps clarify a murky area of the law concerning how much control programmers can exert over their intellectual property once it's been released for free into the so-called "open source" software community.

People are free to use that material in their own products, but they must credit the original authors of the programming code and release their modifications into the wild as well, a cycle that's critical for free software to continue improving.

Because the code was given away for free, thorny questions emerge when a violation has been discovered and someone is found to have shoved the code into their own for-profit products without giving anything back, in the form of attribution and disclosure of the alterations they made.

In the latest case, which involved a computer application that model-train enthusiasts use to program the chips that control their trains, the U.S. District Court in San Francisco ruled that the plaintiff could sue for breach of contract but not copyright infringement.

The distinction is important because it's easier to recover monetary damages in a copyright-infringement case.

Robert Jacobsen, who manages an open source software group that created an application he claims was infringed, sought an injunction against KAM Industries, which makes a competing product. The lower court denied Jacobsen's motion. The appeals court vacated that ruling Wednesday and returned the case to the district court.

"Traditionally, copyright owners sold their copyrighted material in exchange for money," the court said. "The lack of money changing hands in open source licensing should not be presumed to mean that there is no economic consideration, however."