Science Fair Project Encyclopedia
The Vertical (or plumb line) is the direction of the force of gravity. Therefore it is the most important reference line for all creatures and for science and techniques.
The Vertical is a slightly curved line (~1" per km), affected by the terrain and the geology. It intersects the geoid and all equipotentials by exactly 90°.
A pair of angles are said to be vertical if they share the same vertex and are bounded by the same pair of lines but are opposite to each other. If a pair of angles are vertical, then they are (congruent). More precisely, if two lines AB and CD intersect at E, and E is between A and B, and E is between C and D, then angle AEC is vertical to angle BED, and angle AED is vertical to angle BEC.
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