A rack's mounting fixture consists of two parallel metal strips (also referred to as "posts" or "panel mounts") standing vertically. The posts are each 0.625 inches (15.88 mm) wide, and are separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of 19 inches (482.60 mm).
As far as enclosures go, not aware of any standard. I've seen as little as 20.5", out to 23.6", and beyond. It's both company dependent and application dependent.
(The other weird aspect of this is the screw hole spacing. There is the standard space within the 1U height. But, the spacing between rack units is different...well, at least in the audio world. So, if you populate a rack, and you want a gap, and you want all 4 screws, you have to shift 1 full rack unit. A real pain.)
Wapperdude