a ball about an inch in diameter or smaller moved by one digit (almost always the thumb) and the buttons clicked by others, or finger-operated, with a ball over two inches in diameter operated by the middle fingers and the buttons by the thumb and little finger. Users favor one format or another for reasons of comfort, mobility, precision, or because it reduces strain on one part of the hand/wrist. Most, but not all, finger-operated designs are symmetrical in design, making them usable by both hands, while thumb-operated designs are by their nature asymmetric or “handed,” allowing the smallest examples to be held in the air. Thumb-operated trackballs are not generally available in left-handed configurations, due to small demand. Some computer users prefer a trackball over the more common mouse for ergonomic reasons. There seems to be no conclusive evidence from studies performed to determine which type of pointing device works best for most applications. Application users are encouraged to test different devices, and to maintain proper posture and scheduled breaks for comfort. Some disabled users find trackballs easier since they only have to move their thumb relative to their hand, instead of moving the whole hand, while others incur unacceptable fatigue of the thumb. Elderly people sometimes have difficulty holding a mouse still while double-clicking; the trackball allows them to let go of the ball while using the button. At times when a user is browsing menus or websites rather than typing, it is also possible to hold a trackball in the right hand like a television remote control, operating the ball with the right thumb and pressing the buttons with the left thumb, thus giving the fingers a rest. Mobile devices Some mobile devices have trackballs, including those in the BlackBerry range, the T-Mobile Sidekick 3, and many early HTC smartphones. These miniature trackballs are made to fit within the thickness of a mobile device, and are controlled by the tip of a finger or thumb. These have mostly been replaced on smartphones by touch screens, although on the blackberry range they were replaced by an "optical trackball" or "optical trackpad" before later being replaced with touch screens. On mice In lieu of a scroll wheel, some mice include a tiny trackball sometimes called a scroll ball. A popular example is Apple's Mighty Mouse. Mice with a larger trackball on a side may be designed to stay stationary,