Generally, there are two ways people hold their mobile devices when they are using them, including when they are playing games.
Alternative scenarios are possible, but are less common and are used only in specific situations and cannot be considered as a prevalent behavior. For instance, some people play games putting a device on the tabletop (that is more usual for iPad rather than iPhone). Obviously, this is possible only when they are sitting in their living rooms or offices.
The main advantage of such models is free fingers, which can be used for some very shifty multitouch tasks. Another possible player's behavior implies that the two fingers (an index and a thumb) are placed over the screen. It is a little bit ungainly because it is harder to hold a device (especially a full-scale iPad) and operate all fingers; nevertheless, it is worth thinking about.
The only exception is a game that is oriented on the spells or other gaming modes based on single-fingered gestures. Such a game looks very good in a portrait mode, because gesture symbols can be very large and snazzy, especially with some shiny special effect being applied on.
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