A foldable smartphone is a form factor of smartphone, which allows access to a larger tablet-like display or a phablet when needed by unfolding the device, whilst maintaining a similar footprint and functionality to a standard smartphone when folded (typically along a vertical axis). The screen may either wrap around to the back of the device when folded (as with the Royole Flexpai and Huawei Mate X), or use a booklet-like design where the larger, folded screen is located on the interior, and a screen on its "cover" allows the user to interact with the device without opening it (such as the Samsung Galaxy Fold).
Precursors to the concept have used multiple touchscreen panels on a hinge in a similar manner, but the term is currently synonymous with implementations that utilize a flexible display; concepts of such devices date back as early as Nokia's "Morph" concept (2008), and a concept presented by Samsung Electronics in 2013 (as part of a larger set of concepts utilizing flexible OLED displays), while the first commercially-available folding smartphones with OLED displays began to emerge in November 2018.
In 2008, Nokia presented animated concepts of a flexible device it dubbed "Morph", which had a tri-fold design that could be bended into various forms, such as a large unfolded device, a feature phone-sized unit, and a smart wristband. In a 2019 retrospective on the concept, CNET noted that Morph could be considered a forerunner to the first wave of commercially-produced folding phones, as well as a showcase of future possibilities.
In 2011, Kyocera released a dual-touchscreen Android smartphone known as the Echo, which featured a pair of 3.5-inch touchscreens. When folded, the top screen continued to face the user while covering the secondary screen. Two individual apps could be shown on the displays, a single app could span across them, while specific apps also featured "optimized" two-pane layouts. Two years later, NEC released the Medias W in Japan. Unlike the Echo, the secondary screen could be folded behind the phone. The camera rotated with the screen so that the same sensor could face both forward and rear  In 2017, ZTE released the Axon M with a similar hinge to the Medias W. ZTE stated that the more powerful hardware of modern smartphones, and improvements to multitasking and tablet support on Android, helped to improve this experience.
The development of thin, flexible OLED displays enabled the possibility for new designs and form factors. During its Consumer Electronics Show keynote in 2013, Samsung presented several concepts—codenamed Youm—for smartphones incorporating flexible displays. One such concept was a smartphone that could fold outward into a single, uninterrupted tablet-sized display. The first Youm concept to make it to a production model—the Galaxy Note Edge—was a device with a portion of the screen that sloped over the right-hand bezel.
Speculation surrounding the development of folding phones using OLED displays began to emerge more rapidly in 2018. In January 2018, it was reported that LG Electronics had obtained a design patent for a folding smartphone. Later in June, it was reported that Microsoft had been developing a similar device as part of its Surface line, codenamed "Andromeda" (itself a spiritual successor to a dual-screen booklet tablet prototype Microsoft had been exploring in the late-2000's known as Courier), while Samsung was also said to be developing such a device. 
In November 2018, the Chinese startup Royole released the first commercially-available foldable smartphone with an OLED display, the Royole Flexpai. It featured a single 7.8-inch display that folds outwards, leaving the display exposed when folded. Later that month at its developers' conference, Samsung officially teased a prototype of its folding smartphone, which would be produced "in the coming months". The prototype used a booklet-style layout, with an "InfinityFlex" display located on the inside of the device, and a smaller "cover" screen on the front of the device to allow access when the screen is closed. At a concurrent developers' summit, Android VP of engineering Dave Burke stated that the next version of the platform would provide enhancements and guidance relevant to folding devices, leveraging existing features.
In January 2019, Xiaomi CEO Lin Bin published a video on Sina Weibo, featuring him demonstrating a prototype smartphone with two flaps capable of being folded inward. Samsung officially unveiled the Galaxy Fold during its media event at Mobile World Congress in February 2019. Alongside the Galaxy Fold, the convention also saw other foldable phones being unveiled or teased, such as the Huawei Mate X, and TCL presenting various prototype concepts featuring its "DragonHinge" technology (including a braclet-styled device). LG did not unveil a folding device, citing a desire to focus more on re-gaining market share in the smartphone market. It did, however, unveil a "Dual Screen" case accessory for its LG V50 smartphone—a folio-styled case containing a secondary display panel inside.
Other companies have publicly expressed interest in the concept, or have received patents on designs (such as hinge implementations and overall designs) relating to foldable phones. Motorola Mobility announced plans to produce a horizontal folding smartphone reminiscent of clamshell feature phones.
In April 2019, the impending launch of the Samsung Galaxy Fold was met with quality concerns from critics, after widespread reports of review units experiencing varying forms of display failure (in some instances caused by accidental removal of a plastic layer meant to protect the screen in lieu of glass, along with other failures). Samsung indefinitely postponed the device's release, stating that it needed time to investigate the failures, and improve the device's durability. Huawei also delayed its Mate X, with the company citing its desire to take a "cautious" approach due to the Galaxy Fold.
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