29ers or two-niners are mountain bikes and hybrid bikes that are built to use 700c or 622 mm ISO (inside rim diameter) wheels, commonly called 29" wheels. Most mountain bikes use ISO 559 mm wheels, commonly called 26" wheels.

This term is used to refer to the physical specifications of the wheel and the availability of the sizes that will fit a vehicle. So, the fitment specifications for a particular wheel and vehicle include the bolt pattern, the offset, the wheel width and the wheel diameter.

FWD stands for Front Wheel Drive and RWD stands for Rear Wheel Drive. Most wheel catalogs use these acronyms to sort out the wheels available for these two different types of vehicles. The great majority of the time, a FWD wheel will not fit a RWD vehicle and vice versa.