“Sinking primary controls to screen bottom isn’t just a matter of thumb comfort, but also the simple fact that fingers obscure the screen. The bottom is where hovering hands are least likely to cover content…This is a familiar, common-sense layout that applies to most physical devices: iPods, calculators, cell phones, bathroom scales, you name it…[But for Android] adding controls at screen bottom means stacking toolbars on top of each other; alas, tap errors are especially common for stacked controls at the bottom of the screen. In the high-traffic zone where the thumb tends to hover and obscure the view, button collisions are inevitable.”

Link: Designing for touch (netmagazine.com, via)