In an unusual twist, Microsoft and Linux have joined forces in a legal tussle.

The companies jointly sent a letter expressing their common disagreement with advice regarding software law that a group plans to send to judges.

The American Law Institute (ALI), an association that develops documents meant to help guide judges as they make decisions on complicated legal subjects, is presenting a final draft of its guidelines for software contracts on Tuesday during its annual meeting. The document has been in the works since 2004 and if it is approved it will be published.

Despite the differences that most often divide Microsoft, which relies on proprietary software sales for its lifeblood, and the Linux Foundation, which promotes open-source software, the two agree in this situation.

They urge the ALI specifically to clarify a section of its document that concerns warranties on defects in software. The document appears to absolve commercial open-source software from the types of warranties that would be applied to proprietary software.

But because many open-source software providers make money, such as through advertising, it's unclear if such providers would be liable for defects according to the document.

"The principles outlined by the ALI interfere with the natural operation of open source licenses and commercial licenses as well by creating implied warranties that could result in a tremendous amount of unnecessary litigation, which would undermine the sharing of technology," Jim Zemlin, executive director of the Linux Foundation, wrote in a blog post about the letter.

Microsoft and the Linux Foundation are writing and publishing the letter because it is essentially the only way for them to express their opinions about the ALI document. Only ALI members can participate in the process of drafting such documents and there is no public review prior to publication, Zemlin said.

The draft of the document is available for download for US$45.

Microsoft and the Linux Foundation are asking the ALI to delay adoption of the principles to allow more time for input from interested parties.

They both recognize how unusual it is to be on the same side of such a debate.

"The mere fact that the Linux Foundation and Microsoft are joining forces may be viewed by some as remarkable," wrote Horacio Gutierrez, deputy general counsel for Microsoft in a blog post.

"But there is a wide range of issues that affect all software developers alike."