How does the adjective vertical differ from other similar words?
The words perpendicular and plumb are common synonyms of vertical. While all three words mean "being at right angles to a base line," vertical suggests a line or direction rising straight upward toward a zenith.
the side of the cliff is almost vertical
When could perpendicular be used to replace vertical?
The words perpendicular and vertical are synonyms, but do differ in nuance. Specifically, perpendicular may stress the straightness of a line making a right angle with any other line, not necessarily a horizontal one.
the parallel bars are perpendicular to the support posts
When is it sensible to use plumb instead of vertical?
While the synonyms plumb and vertical are close in meaning, plumb stresses an exact verticality determined (as with a plumb line) by earth's gravity.