Well, the biggest misconception about MacOS X is that it was built on top of FreeBSD. It's actually built on top of Mach that they perfected at NeXT (Avie Tevan

Well, the biggest misconception about MacOS X is that it was built on top of FreeBSD. It's actually built on top of Mach that they perfected at NeXT (Avie Tevanian was one of the creators of Mach at CMU). FreeBSD was added as one of the sublayers.