Your faithful author spent many years of his misspent youth with a svelte Casio calculator watch strapped to his wrist before finally retiring it in 1985. The problem wasn't just its tiny controls or its dampening effect on my prom king prospects. The buttons were too close together. Forget factorials or logarithms, the hardest thing about doing math on this thing was just hitting the right button. You'd aim for a five, but come up with a two or an eight, who knows—it was more wheel of fortune than calculator. Button size, in other words, isn't the only determining factor of tap accuracy; you have to consider spacing, too.
The iPhone's 320 × 480 pixels make for cozy quarters, and you'll inevitably be tempted to deal with that challenge by crowding the interface. "I'll just nudge these a little closer. I'll just add one more button to this toolbar." To quote a popular phrase of the calculator-watch heyday: "Just say no."
Figure 3-10. The iPhone's standard tab bar ensures comfortable spacing by limiting app designers to five tabs—no crowding allowed. If you add more than five, the tab bar displays only four of them, adding the More icon which takes you to a screen with additional options, as shown at left in the New York Times app. Apple's guidelines suggest limiting toolbars to just five icons, as shown at right in Safari.
Once again, Apple's standard controls help you do the right thing. For example, when your app uses a standard tab bar at screen bottom to switch between modes or views, the operating system automatically spaces them out for you and limits you to a maximum of five tabs. Toolbars don't impose the same automatic limit, but Apple nevertheless recommends that you limit toolbar icons to five, too. (You'll learn all about tab bars and toolbars in Tab Bar: What's on the Menu? and The Toolbar respectively.) In both cases, the screen's 320-pixel width is technically large enough to cram up to seven 44-pixel icons, but they'd bump right up next to each other, introducing the calculator-watch problem for your audience.
It's especially important to give fingers some breathing room at the bottom of the screen. Usability testing reveals this to be a clumsy area prone to mistaps when targets are placed too close to an app's tab-bar navigation. The frustration is compounded by the fact that accidentally tapping a tab bar icon often takes you to an entirely new screen. If you must place targets near the tab bar or toolbar, make sure they're large enough to hit easily. (See how USA Today worked around this problem in Big Problem with Tiny Buttons.)
Figure 3-11. Call Global App and Skype both include phone keypads with buttons that press right up against the tab bar. Call Global (left) makes things especially difficult by making the adjacent buttons narrow and tough to hit, with frequent missed taps. When you want to see your balance, a mistaken tap sweeps you away to the app's World Call screen. In Skype (right), the problem is less pronounced because the big buttons are tough to miss.