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.

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

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

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/vertically-opposite-angles.html

(in fact they are congruent angles). Have a play with them yourself. Notice how
the 4 angles are actually two pairs of "vertically opposite angles": Reset.

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 ...

mathbitsnotebook.com/Geometry/SegmentsAnglesTriangles/SATAnglePairs.html

If two congruent angles form a linear pair, the angles are right angles. If two
congruent angles ... are a linear pair). Vertical angles are always equal in
measure.

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

Proving that vertical angles are equal. ... Vertical angles are congruent proof ... --
so, angle DBA-- then I'd prove that vertical angles are always going to be equal, ...

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.algebralab.org/lessons/lesson.aspx?file=geometry_anglescomplementarysupplementaryvertical.xml

Vertical angles are two angles whose sides form two pairs of opposite rays. ... It is
always true that the two acute angles in a right triangle are complementary.