Tipping the screen on its side causes a moment of disorientation, and it's your job as a designer to steady that temporary interface vertigo. By choosing a point of gravity for your screen, you can keep the user grounded even as the screen's contents tumble into new locations. It's a good practice to use the screen's top edge as the anchor position. Any text that appears at the top of the portrait screen should likewise appear at the top of the screen's landscape version (instead of anchoring the screen around whatever content is at the center, for example).
Certain screens defy alternate orientations because their precise and persistent layout is itself a usability aid. If Apple let you flip the iPhone Home screen on its side, for example, the icons would scatter into new positions, shifting out of their familiar places and creating a cognitive jumble for you to hunt through. That's the same reason that Facebook's Home-screen grid of icons (page 237) is the app's one portrait-only screen. "I didn't want to change the relative positions of the icons by shifting it from a 3 × 3 grid to 4 × 2 or something like that, but that was the only way it looked good," says developer Joe Hewitt, who created the Facebook app's grid design. "The problem is that your whole layout gets changed. You're used to the buttons being in a certain grid, and in landscape you'd have to go searching for them. It just didn't feel right, so I didn't do it. When you rotate, you always have to make sure everything is lined up in a place where people can easily find it again."
While your app's pixels might be under your control, the person holding it is not. People use their iPhones anywhere and everywhere, and that means all kinds of positions, too. Lying down, leaning over, shifting in your seat—our shimmies in the physical world trigger accidental (and annoying) screen rotations all the time. A comfortable position for bedtime browsing sometimes involves holding the phone in a distinctly awkward way just to keep the orientation steady. Screen rotation is great except when it happens by surprise.
The iPhone's big brother, the iPad, has a physical switch to lock rotation, and beginning with iPhone OS 4.0, there's a software button to do the same on the iPhone. Even with these systemwide switches to keep your baby from rocking, it's not a bad idea for individual apps to add their own rotation lock, too. This is an especially good idea for ebook readers, other long-form reading apps, and any app where someone is likely to spend at least several minutes at a time poring over the screen. Those are the situations that invite reclining, and when you put your feet up and get comfortable that's when the accidental screen tilt inevitably arrives. If your app fits those conditions, consider adding a setting or button that locks your app's orientation in place. The iPhone might not have a fixed idea of "up," but your audience does; give them the ability to point the way.
