While including PDA applications (address book, calendar, to-do lists, etc.) as well as telephone capabilities on newer models, the BlackBerry is primarily known for its ability to send and receive e-mail wherever it can access a wireless network of certain cellular phone carriers. It has a built-in QWERTY keyboard, optimized for thumbing, the use of only the thumbs to type. System navigation is primarily accomplished by a scroll ball in the middle of the device (older devices used a track wheel on the side). Some models (currently, those manufactured for use with iDEN networks such as Nextel and Mike) also incorporate a Push-to-Talk (PTT) feature, similar to a two-way radio. (*)