How to Check Uni body for Straightness

I would like to clear something up here. There is some really good information here that I agree with BUT...
Technically speaking, the term "twist" only exists in the center section of the automobile.
If the center section is square then every thing else as far as datum is concerned is called high rail or low rail.
You could have what might be considered as a high rail but it would be corrected by straightening the center section that has a twist. This is why we work from the center section.