#677 – Why the Standard Mouse Wheel Delta is 120

For a standard Microsoft mouse, you’ll notice that the value of the MouseWheelEventArgs.Delta parameter is always either 120 when you turn the mouse wheel forward one click or -120 when you turn the mouse wheel backwards one click.  You might wonder why the value of 120 is used, rather than just 1 and -1 values.

The larger number for the mouse wheel delta is used for a typical mouse so that other manufacturers of mouse devices can include a mouse wheel with more precision, which could then return a smaller value for each delta.  For example, there might be twice as many click detents on a device, which could then return a delta value of 60.

The value of 120 can be evenly divided by 2, 3, 4 or 5.  This gives a mouse manufacturer more options in dividing up the current coarser clicks into an even number of higher resolution clicks.