5-way navigator

From TreoCentral

The 5-way navigator is the big thumb button on the center of the Treo.

Each of the applications built into the Treo 600 series is five-way enabled which means navigating through menu options, on-screen commands, links for web pages, messages, etc. can all be done using one hand-without the need for a stylus. Additionally, the main application buttons and the five-way button are in the middle of the device for easier one-handed use.