With several mobile device platforms in wide use, the designer's job is more complex than ever. Suzanne Ginsburg sorts it all out, proposing several approaches and giving examples and tradeoffs of each. Should you design a native app only for the iPhone? A web app that works on Android and Windows phones, too? Or should your design fall somewhere between, targeting only a few platforms? Or take a hybrid approach? Suzanne gives guidelines to help you decide and describes tools that can ease the designer's task.

Sketching is a useful, low-cost design technique, whether the target is one platform or all platforms. Sketching can reveal differences in platforms that are best considered early, such as Android's hardware "Back" and "Menu" buttons, not present on the iPhone.  Low-cost prototyping is possible, too, using simple web or Keynote/Powerpoint mock-ups.

Suzanne gives analogies to three main approaches to mobile app design:

• One Trick Pony: Design a native app for just one platform.
• OK Corral: Design a native app for two or a few platforms.
• Trojan Horse: Design a cross-platform app, a hybrid between native and web apps.

Plus a fourth approach, appropriate for many projects: Design a cross-platform web app, especially if the app doesn't require access to all of the device's hardware features.

Suzanne explains the differences between native apps and web apps, listing the circumstances in which each is superior. Web apps, accessible via browser and automatically updating, are relatively inexpensive to create, while native apps and their updates must be downloaded, though they have greater access to hardware and content stored on the device.

For each approach, Suzanne shows example apps and reviews tools that can make the designer's job easier.