
In geometry, a surface S is ruled (also called a scroll) if through every point of S there is a straight line that lies on S. The most familiar examples (illustrated here in three-dimensional Euclidean space) are the plane and the curved surface of a cylinder or cone. Other examples are a conical surface with elliptical directrix, the right conoid...
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http://en.wikipedia.org/wiki/Ruled_surface

A surface that is built up from an infinite number of perfectly straight lines. A cylinder, for example, is a ruled surface of parallel straight lines. A cone is a ruled surface of straight lines that meet at the apex of the cone. Also known as scrolls, ruled surfaces have been studied for centuries...
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http://www.daviddarling.info/encyclopedia/R/ruled_surface.html

A surface formed by moving a straight line.
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https://www.encyclo.co.uk/local/20687

a surface that can be generated by a straight line, as a cylinder or cone.
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https://www.infoplease.com/dictionary/ruled-surface
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