The MacBook Pro (sometimes abbreviated MBP) is a line of Macintosh portable computers introduced in January 2006 by Apple Inc., and now in its third generation. Replacing the PowerBook G4, the MacBook Pro was the second model, after the iMac, to be announced in the Apple–Intel transition. It is also the high-end model of the MacBook family and is currently produced with 13- and 15-inch screens, while a 17-inch version was also available in the past. The first generation MacBook Pro appeared externally similar to the PowerBook G4, but used the Intel Core processors instead of PowerPC G4 chips. The 15-inch model was released in January 2006, a 17-inch model in April, both of which received several updates and Core 2 Duo processors later in the year. The second model, known as the “unibody” model, has a more tapered design and a casing made from a single block of aluminum. It debuted in October 2008 as the 15-inch MacBook Pro and the 13-inch aluminum unibody MacBook. The following January brought the design to the 17-inch model, along with the built-in battery that joined the rest of the MacBook Pro line in June, including the 13-inch model which Apple absorbed into the MacBook Pro line. Subsequent updates brought upgraded Intel Core i5 and i7 processors and introduced Intel’s Thunderbolt technology. Apple released the third generation of MacBook Pro in June 2012 with a 15-inch screen. At the same time, slightly updated versions of the previous generation 13- and 15-inch unibody models were announced and sold in parallel, and the 17-inch variant was discontinued. While dimensionally smaller than its predecessor, the similarly styled third generation model retained a unibody design. The most substantial differences in the third generation MacBook Pro are the fitting of a significantly higher resolution Retina display, the elimination of the optical drive, and replacement of hard disk drives with solid-state drives. A 13-inch third generation MacBook Pro was released in October 2012.