The MacBook is a line of Macintosh notebook computers designed, manufactured and sold by Apple Inc. from May 2006 to February 2012. A new line of computers by the same name was released in 2015, serving the same purpose as an entry-level laptop. It replaced the iBook series and 12-inch PowerBook series of notebooks as a part of Apple's transition from PowerPC to Intel processors. Positioned as the low end of the MacBook family, below the premium ultra-portable MacBook Air and the powerful MacBook Pro, the MacBook was aimed at the consumer and education markets. It was the best-selling Macintosh ever. For five months in 2008, it was the best-selling laptop of any brand in US retail stores. Collectively, the MacBook brand is the "world's top-selling line of premium laptops." There have been four separate designs of the MacBook. The original model used a combination of polycarbonate and fiberglass casing which was modeled after the iBook G4. The second type was introduced in October 2008 alongside the 15-inch MacBook Pro; the MacBook shared the more expensive laptop's unibody aluminium casing, but omitted FireWire. A third design, introduced in late 2009, had a polycarbonate unibody casing. On July 20, 2011, the MacBook was discontinued for consumer purchase as it had been effectively superseded by the MacBook Air which had a lower entry price. Apple continued to sell the MacBook to educational institutions until February 2012.