Antiparallel lines

are antiparallel with respect to a given line

if they each make congruent angles with

are antiparallel with respect to another pair of lines

if they are antiparallel with respect to the angle bisector of

In an oblique cone, there are exactly two families of parallel planes whose sections with the cone are circles.

One of these families is parallel to the fixed generating circle and the other is called by Apollonius the subcontrary sections.

[1] If one looks at the triangles formed by the diameters of the circular sections (both families) and the vertex of the cone (triangles ABC and ADB), they are all similar.

That is, if CB and BD are antiparallel with respect to lines AB and AC, then all sections of the cone parallel to either one of these circles will be circles.

This is Book 1, Proposition 5 in Apollonius.

Lines and are antiparallel with respect to the line if they make the same angle with in the opposite senses.
Two lines and are antiparallel with respect to the sides of an angle if they make the same angle in the opposite senses with the bisector of that angle.
Given two lines and , lines and are antiparallel with respect to and if .
In any quadrilateral inscribed in a circle, any two opposite sides are antiparallel with respect to the other two sides.
red angles are of equal size, ED and the tangent in B are antiparallel to AC and are perpendicular to MB
A cone with two directions of circular sections
Side view of a cone with the two antiparallel directions of circular sections.
Triangles ABC and ADB are similar