Fuji Track / Fuji Track Pro / Fuji Track Classic

As far as I understand it, you can put a freewheel on a fixed hub but you can't put a fixed cog and lockring on a freewheel threaded hub. Double fixed could therefore be double free if you so chose. Some hubs I believe come with a fixed cog and lockring thread on one side and a freewheel thread on the other, this would be the fixed/free variety.

I've never got why all hubs (geared exluded obviously) aren't fixed/fixed for this reason.