[Weightshift]

Weightshift:

You are correct in that 'vertical' means 'perpendicular' or 'at right angles to,' and I encourage you to use that term if you prefer.

Actually I made a mistake. According to the dictionary..
perpendicular: At right angles to plane of horizon.

So 'perpendicular' means 'vertical'. I always thought it meant at right angles to anything

It seems that others are confused too:

http://dictionary.reference.com/search?q=right%20angle
"An angle formed by the perpendicular intersection of two straight lines; an angle of 90°.

However..
http://thesaurus.reference.com/search?q=perpendicular
Synonym: vertical (Alan: no mention of 90 degrees)
Antonym: horizontal

We are incorrect in saying that vertical (or perpendicular) means at right angles to. Vertical (or perpendicular) is DEFINED as being at right angles to plane of horizon (ONLY AT RIGHT ANGLES TO PLANE OF HORIZON) not at right angles to anything we choose.

The term is, as you say, "absolute" in that it defines a 90 degree relationship between, e.g., two surfaces.

"Vertical" is absolute in that it is FIXED at right angles to the plane of the horizon. If we wish to define a relationship between two planes at right angles to each other, we must say that they are "at right angles to each other" or "have a 90 degree relationship".

Of all the myriad of possible planes, Vertical and Horizontal planes are special, absolute, cases (defined by Man), as all other (angled) planes fall within these two limits.

'Horizontal', 'Vertical' and 'Angled' are all adjectives but the first two are also nouns e.g. "..to the Vertical"

However, that relationship can be with any given plane, not just the horizontal. Which, of course, makes it "relative."

And that is the way the term is used in TGM

So unfortunate.
No wonder I couldn't understand some of the text!