There was a long debate about how many hardware buttons are required for Windows Phone 7. There were those betting on three and those who thought of the number five. I don't remember any other number being speculated. Well, actually six hardware buttons are required by Windows Phone 7.
1. Power/Sleep: this button has several applications: it Boots the phone when powered off, locks the Device and turns off Screen when normal operation is on, turns screen off when the screen is locked, turns the screen on when the screen is off and the Device is locked and turns the phone off via either Software or Hardware (depending on the implementation of the period of time it is pushed).
2. Volume Up and Down: will adjust the volume of either the in-call conversation volume (if a phone call is active), or else the overall device volume.
3. Camera: this has two operation modes like with the majority of cameras, it will support full and half presses. Half button press will focus the camera and the full press will take the picture. Also, pressing the Camera button will start the Camera application.
4. Back: will take the user one step/page back to the previous screen/page within the same application or between applications. Other functionalities of the Back button include closing menus, dialogs, navigating to a previous page, exiting a search operation, or even switching applications.
5. Start: when the Start button is pressed, all the applications will receive a Pause command and the Device will present the Start in the phone user interface.
6. Search: will launch Bing search to find content anywhere on the Device.