A 29" mountain bike uses 700c rims with knobbies just as fat as 26" knobbies. The name 29" comes from the fact that the outside diameter, with the fat tire installed, is appox. 29." A frame designed for 26" knobbies won't accept a 29" mountain bike tire. 29ers are becoming popular, the Gary Fisher brand is currently producing these, and they can be seen at any Fisher dealer. It's a stark difference when you put a 26" mountain bike tire/wheel next to a 29" mountain bike tire/wheel. The 29" bikes are very versatile, too, because you can install 700c road tires on the rims.