Why do most iPhone apps not work upside down?

Question

It's capable of detecting it so why do most apps not work upside down?

Answer 1

This is because there are many situations where the device will appear upside down to the accelerometer without being upside down from the user's view.

For example when the user looks away from the phone while holding it in one hand, they will most likely lower their arm and relax their hand which is then bent forward due to the phone's weight causing the phone to be upside down. Rotating the view here would be unexpected and would cause annoyance because the view needs to be rotated back when the user directs attention to the phone and lifts it up again.

Furthermore, by not responding to the upside-down orientation, apps are usable while laying down in bed on your side.

Note that things are different on the iPad, which is intended to be a collaborative device. It's nice to be able to hand it over to someone else without the need of rotating it.

Answer 2

The answer can be found in the iOS human interface guidelines:

Think Top Down

The top of the screen is most visible to people, because they tend to interact with the device by holding the device in the following ways:

In their nondominant hand (or laying it on a surface), and gesturing with a finger of the dominant hand In one hand, and gesturing with the thumb of the same hand Between their hands, and gesturing with both thumbs Put the most frequently used (usually higher level) information near the top, where it is most visible and easy to reach. As the user scans the screen from top to bottom, the information displayed should progress from general to specific and from high level to low level.

For example, in a game, the most important action can take place in the top half of the screen. This leaves the bottom half of the screen for supplementary information and for controls users can tap without obscuring their view.

And although this is referring to the views, you can extrapolate this to the device orientation.

Still if you think your user might find some use on the upside-down orientation you should always add it.

I don't know but if you get a call during an upside-down orientation operation you would have to rotate 180º (pi radians) the phone to answer which wouldn't be cool for some users.

I like to support upside-down device orientation for curious users to find it as a silly easter egg :P