Stallman and his crew , but there was one missing piece: a kernel, which provides resources from the hardware to programs that run on the computer. This gap was filled by Torvalds in 1991 when he developed Linux, a Unix-like kernel.

Systems using the Linux kernel are usually called just "Linux," but Stallman has fought for years to get people to use his preferred term, "GNU/Linux."

Stallman "wanted to make sure GNU got proper credit," of Novell, who created the free software program GNOME but has been criticized by Stallman for partnering with Microsoft and selling proprietary software. "When Linux came out, Richard didn't take it very seriously for a while, and he kept working on his own kernel. It was only when Linux took the spotlight that he felt, to some extent, his project had not been given enough credit. The problem is, what happened at the time was there was a new community that was created out of the blue that wasn't necessarily aligned with GNU."

The GNU kernel, called Hurd, is still "under active development," .

Torvalds' contribution to free software will be widely celebrated this year during the 20th anniversary of the Linux kernel. But Stallman won't be one of his cheerleaders, and it's not just because of the naming dispute.