I just bought a new mouse - just a cheapie usb wheel mouse but it has an extra 'thumb' button which I thought I might as well set up to do something useful.

I used 'xev' to determine the numbers that the OS had assigned to the buttons, and I found it wasn't quite what I was expecting:
left = 1
wheel-as-button = 2
right = 3
wheel-up = 4
wheel-down = 5
thumb = 8

I thought the right button would be '2' and the 'wheel-as-button' would be 3 - that's just how I've always seen them referred to in explanations, and I thought it was standardised. Not a big deal - the context menus still pop up when I click the right button so it all works ok. (And I just discovered the wheel-as-button does a 'paste' operation when it's clicked. Weird - I've never noticed that before...)

But the thing that I find particularly odd: why is the thumb button assigned as '8'. What happened to '6' and '7'?

Does the operating system assign these numbers, or are they built into the hardware in the mouse itself?

Again, no big deal, I know I can remap the buttons if I need to, but I was just curious as to how this all works and google didn't help.

I believe the button codes are configured by the x input driver that controls it, either the old mouse driver, or, in most distros these days, evdev.

There is a standard for input devices that's supposed to ensure that the basic buttons at least are always configured the same. But I believe there are quite a few oddball devices out there, especially among the cheap stuff, that while supposedly conforming to standards, in fact don't quite do what you expect them to. You seem to have found one of those.

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Thanks for the response.

Yes you're probably right about this being a non-standard mouse - it's a cheapo K-Mart brand, so I doubt if standards compliance or quality control was high on the manufacturer's priority list .

I had a look at xorg.conf and there was nothing in there about button numbers, so either they're stored elsewhere or they're assigned at boot time. I did a brief search but I couldn't find any more info, so I'll be content with knowing the mouse works ok - and I just used 'b:8' to assign a function to the thumb button in xbindkeys.