en.wikipedia.org/wiki/Transversal_(geometry)

In geometry, a transversal is a line that passes through two lines in the same
plane at two ... A transversal that cuts two parallel lines at right angles is called a
... pair of corresponding angle...

www.mathopenref.com/anglesvertical.html

As can be seen from the figure above, when two lines intersect, four angles are
formed. Each opposite pair are called vertical angles and are always congruent.

www.mathopenref.com/congruentangles.html

Angles are congruent if they have the same angle measure in degrees. They can
be at any orientation on the plane. In the figure above, there are two congruent ...

www.mathplanet.com/education/pre-algebra/introducing-geometry/angles-and-parallel-lines

Vertical angles are always congruent, which means that they are equal. ... that
are on the opposite sides of the transversal are called alternate angles e.g. 1 + 8.

study.com/academy/lesson/types-of-angles-vertical-corresponding-alternate-interior-others.html

They are defined as a pair of nonadjacent angles formed by only two intersecting
lines. They are known as 'Kissing Vs' and always have congruent measures.

www.mathsisfun.com/geometry/vertically-opposite-angles.html

(in fact they are congruent angles), vertically opposite a and b ... Angles a° and c°
are also vertical angles, so must be equal, which means they are ... Language
Note: "Vertically Opposite Angles" are usually called Vertical Angles in the US.

math.stackexchange.com/questions/286114/explain-why-vertical-angles-must-be-congruent

Jan 24, 2013 ... Explain why vertical angles must be congruent .... The non-adjacent angles are
called vertical or opposite . Also, each pair of adjacent angles ...

www.khanacademy.org/math/geometry-home/geometry-angles/geometry-vert-comp-supp/v/proof-vertical-angles-are-equal

What I want to do is if I can prove that angle CBE is always going to be equal to
its vertical angle --so, angle DBA-- then I'd prove that vertical angles are always ...

www.reference.com/math/vertical-angles-congruent-e2e1fa473d7df470

In mathematics, vertical angles are always congruent. Vertical angles are the
angles ... when two lines intersect. They are called vertical because the angles
share.

www.math.com/school/subject3/lessons/S3U1L5DP.html

Vertical angles are pairs of angles formed by two intersecting lines. ... in four
corresponding pairs that have the same measure, so therefore are congruent. ...
area between the two lines that are cut by the transversal, are called interior
angles.