In three-dimensional space, a regulus R is a set of skew lines, every point of which is on a transversal which intersects an element of R only once, and such that every point on a transversal lies on a line of R.

The set of transversals of R forms an opposite regulus S. In the union R ∪ S is the ruled surface of a hyperboloid of one sheet.

Any 3 skew lines generate a pair of reguli:H. S. M. Coxeter (1969) Introduction to Geometry, page 259, John Wiley & Sons

The set of lines that intersect all 3 of them sweeps out a quadratic surface. This ruling of this quadratic surface is the regulus. The set of lines that intersect all lines in the regulus is the complementary regulus or associated regulus, by Gallucci's theorem.

Any 3 lines in a regulus generate the complementary regulus, and vice versa. The regulus surface is the unique quadratic surface that contains these 3 lines. The pair of reguli sweep out the same surface, showing that it is a doubly ruled surface.

According to Charlotte Scott, "The regulus supplies extremely simple proofs of the properties of a conic...the theorems of Chasles, Brianchon, and Pascal ..."Charlotte Angas Scott (1905) The elementary treatment of the conics by means of the regulus, Bulletin of the American Mathematical Society 12(1): 1–7

In a finite geometry PG(3, q), a regulus has q + 1 lines.Albrecht Beutelspacher & Ute Rosenbaum (1998) Projective Geometry, page 72, Cambridge University Press For example, in 1954 William Edge described a pair of reguli of four lines each in PG(3,3).W. L. Edge (1954) "Geometry of three dimensions over GF(3)", Proceedings of the Royal Society A 222: 262–86

Robert J. T. Bell described how the regulus is generated by a moving straight line. First, the hyperboloid is factored as

Then the following two systems of lines, parametrized by λ and μ satisfy that equation: and

No member of the first set of lines is a member of the second. As λ or μ varies, the hyperboloid is generated. The two sets represent a regulus and its opposite. Using analytic geometry, Bell proves that no two generators in a set intersect, and that any two generators in opposite reguli do intersect and form the plane tangent to the hyperboloid at that point. (page 155).Robert J. T. Bell (1910) An Elementary Treatise on Co-ordinate Geometry of Three Dimensions, page 148, via Internet Archive

See also Spread (projective geometry)

References

H. G. Forder (1950) Geometry, page 118, Hutchinson's University Library.