The fall line is a line on a map, usually approximately parallel to the coastline, which is drawn between the waterfalls or shoals which first block navigation from the ocean up the rivers. On one side of the fall line is a coastal plain; on the other is hilly or mountainous country.

Settlements often are established right around the fall line so that items being shipped upriver can be unloaded there, since it's as far as they can go by that route. For example, a map of the fall line in the U.S. South nearly connects Baltimore, Maryland; Washington, DC; Richmond, Virginia; Raleigh, North Carolina; Columbia, South Carolina; Augusta and Macon, Georgia; and Tuscaloosa, Alabama.

A fall line is also the path that an unobstructed object takes down an inclined plane, or along which an object slides when suspended from a catenary, or the path of a pendulum. Deviation from the fall line consumes energy, usually applied as friction.

For example, skiers who are trying to be badass will take as vertical a fall line as possible down a slope, but when they're going as fast as their nerves can stand they will apply the edges of their skis to push themselves off the fall line and slow down.

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