Google requires [for market access] “all hardware components have the same software APIs as defined in their SDK to be compatible,” says Kitagawa. “This provides consistency to third-party developers that access these components,” he says.

That means devices always need to have some key components, such as touchscreens with a specified minimum resolution, a certain number of navigation keys, Wi-Fi, camera and accelerometer. Devices such as the Alex e-reader or a digital picture frame don’t conform to these requirements.