No problem. I could be a wisea$$ and answer they are just wide enough to fit the various rail buttons that are made to fit them, but that just wouldn't be me .
Thanks to Todd, this thread got me thinking about rails vs. lugs. So I ordered some conformal rail guides from ASP rocketry for a 2.5/2.6 inch diameter tube. These are linear rather than buttons. For some inexplicable reason, I don't like the way buttons look in spite of it all. No accounting for taste evidently.