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

In geometry, a transversal is a line that passes through two lines in the same
plane at two ... If the two angles of one pair are congruent (equal in measure),
then the angles of each of the other ....

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

When two lines intersect they form two pairs of opposite angles, A + C and B + D.
... Vertical angles are always congruent, which means that they are equal.

www.mathsisfun.com/geometry/congruent-angles.html

Congruent Angles have the same angle (in degrees or radians). That is all. ... the
same angle. Congruent - why such a funny word that basically means "equal"?

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.

study.com/academy/lesson/vertical-angles-in-geometry-definition-examples-quiz.html

Congruent Angles: Definition & Examples .... Vertical angles are a pair of non-
adjacent angles formed when two lines intersect. We see ... In addition, angles
that form a linear pair are also supplementary, so their sum is always 180
degrees.

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

Jan 24, 2013 ... Two intersecting lines form two pair of congruent vertical angles. If the vertical
angles of two intersecting lines fail to be congruent, then the two ...

www.math10.com/en/geometry/angles/angles.html

Angles between two parallel lines that are cut by a third line. ... Recall that all
pairs of vertical angles are congruent. Supplementary pairs: angle 1 and angle 2

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.

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 ...