These words refer to a line that stands so as to form a 90-degree angle with a flat, level surface. Vertical may stress an exact 90-degree alignment: Because the floors slanted, a level had to be used to make sure the supports for the bookcases were vertical . But the word is often used to indicate approximate alignment as well: the vertical shafts of young poplars. It can apply, even more vaguely, to any upward motion: aircraft capable of vertical take-off; setting off a vertical escalation of share prices.

In case where vertical might suggest an approximation, perpendicular itself stresses an exact 90-degree angle: vertical rays of sunlight falling across the city’s perpendicular canyons of glass and steel. Perpendicular can apply to upward motion, but may be preferred to vertical for describing sharp downward motion: climbers trying to scale the almost perpendicular mountainside; tons of water falling in a perpendicular line from the cliff’s edge.

Upright is less formal than either vertical or perpendicular and stresses an approximate rather than exact up-and-down alignment, with an implied comparison to something that leans or has fallen: several columns among the ruins that still stood defiantly upright . In figurative uses, moral decency or resoluteness may be suggested. Plumb is related to a context of building or carpentry, in which it suggests that an exact perpendicular alignment has been achieved by use of a plumb line: tapping the bottom of the four-by-two until it was plumb .