When designing for a mobile game, it's important to consider how the player will hold the device. You don't want to put your UI in areas that will be difficult for the player to reach. Players tend to prefer to hold and play with one hand. Not all games allow for this, but if possible, you want to allow your players to do so. How do you know if your UI is in an area reachable by the thumb? Put the UI in the thumb zone! Essentially, the thumb zone is the area of the phone that is comfortable for the player to reach when holding with one hand. You can find the thumb zone on your particular phone by holding the phone and easily moving your thumb around without having to move your hand.
The following blog post offers a really great explanation of the thumb zone, along with a handy (no pun intended) template for finding the thumb zone on various devices: http...
