I would say three: left, right and middle. The middle button (which I use quite a lot because it does gpm paste) is the scroll wheel. But while browsing my Xorg.0.log file, I found these lines:

[ 75.259] (**) evdev: Logitech USB Optical Mouse: Device: "/dev/input/event4"
[ 75.310] (--) evdev: Logitech USB Optical Mouse: Vendor 0x46d Product 0xc077
[ 75.310] (--) evdev: Logitech USB Optical Mouse: Found 12 mouse buttons
[ 75.310] (--) evdev: Logitech USB Optical Mouse: Found scroll wheel(s)
[ 75.310] (--) evdev: Logitech USB Optical Mouse: Found relative axes
[ 75.310] (--) evdev: Logitech USB Optical Mouse: Found x and y relative axes
[ 75.310] (II) evdev: Logitech USB Optical Mouse: Configuring as mouse
[ 75.310] (II) evdev: Logitech USB Optical Mouse: Adding scrollwheel support
[ 75.310] (**) evdev: Logitech USB Optical Mouse: YAxisMapping: buttons 4 and 5
[ 75.310] (**) evdev: Logitech USB Optical Mouse: EmulateWheelButton: 4, EmulateWheelInertia: 10, EmulateWheelTimeout: 200

If evdev has found 12 buttons, where (physically) are the other 9? Does it regard wheel rotations as presses of buttons 4 & 5, since it has mapped those buttons to YAxisMapping?

the scrollwheel already has 3 buttons: up, down, and press.
i think sometimes the scrollwheel can be pressed sideways, too.
still doesn't amount to 12 though.

Actually, what is identified is the base interface to the mouse. The ACTUAL buttons that are connected are determined by the housing. The circuit board inside the mouse is generic, and has potential connections to many different buttons, but only a few are actually used by your mouse.

The "other 9" are just not connected.

This happens in a lot of places - For instance, a SCSI controller can have 16 targets... but each target may have 8 units. I've never seen more than one... but still, where are the other 7 units? Never connected...

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One computer I had had ghost video outputs ... and one of them was default. Took awhile before I figured out why there is no picture.

Ah! So there really are twelve available connections in my mouse but only five of them are in use, and the wheel uses three of those five. When I rotate the wheel, the click that I hear is button 4 or 5. When I press it, that's button 3.

Very well explained.

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