Headset advice please

goshow Posts: 61
edited November 2012 in Workshop
I'm in the process of building a carbon road bike and am now at the stage of ordering a headset.

I'm a massive fan of anything made by hope so thought I'd try and identify the correct headset. They don't make one for my bike.

I've pretty much identified that I need a campy compatible headset due to the cups having 45 degree seats top and bottom and measuring 41.8mm O/D.

The question i have is; I see ritchey do a compatible headset but they come in 10mm, 15mm and 25mm campagnolo sizes. What do these measurments represent?