Apple’s iPad Human Interface Guidelines say the following:

Whenever possible, add a realistic, physical dimension to your application. The more true to life your application looks and behaves, the easier it is for people to understand how it works and the more they enjoy using it.

When I first saw these guidelines, I was skeptical. There have been many UIs in the past that have employed realism and physical dimension only to come off feeling a bit cheesy. However, I’ve been working on some iPad designs this past week and it finally occurred to me why physicality is so important to the iPad experience and why other attempts at physicality and realism have fallen flat (pardon the pun).

The reason now seems obvious (I’ll readily admit that I’m just a little slow sometimes). In a touch interface, physicality is critical for an intuitive UI. There are no mouse over tooltips to give you additional clues about how to interact with an object. As a result, users must be encouraged to explore gestures that mimic real life. For example, take the spread/pinch gesture demonstrated in iPad’s Photo app:

Pinch and spread stacks of photos

This gesture works because the mental construct is a physical stack of photos. Clearly, this same gesture would not make sense in a list or an entry in your address book.

But the behavior of the pinch/spread gesture is also contextual. This same gesture performs a zoom action when viewing a single photo or a map, for example. So the realism and physicality provide an affordance for gestures while they also provide the appropriate context for the action performed.