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27.5 mountain bikes, also called tweeners, are mountain bikes which use a wheel that is approximately 27.5 inches in diameter with a large volume ISO 56-584 (27.5 x 2.25) mountain bike tire installed in a ISO 584 mm rim. The wheel size is also known as ISO 584 mm and 650B. The "650B" is used as a "marketing term" by some manufacturers for their 27.5", which has traditionally been a designation for a 26 inch diameter (ISO ~ 40-584 demi-ballon tire) using the same ISO 584 mm rim used by French tandems, Porteurs and touring bicycles.
The 27.5 inch are seen as a compromise between the two existing standards of the original 26 inch and recently emerged 29 inch mountain bikes. They were pioneered by Kirk Pacenti in 2007, and as of 2013, at least 10 companies are launching models with 27.5 inch wheels, and parts manufacturers are following suit.
Nino Schurter won the World Cup event at Pietermaritzburg, South Africa, and placed second in the Olympics in 2012 on 27.5 inch wheels.