If two parallel lines are transected by a third line,
the angles which are inside the parallel lines and on alternate sides of the transverse line are called alternate interior angles.
If two parallel lines are transected by a third line,
these angles are called corresponding angles.
If two parallel lines are transected by a third line,
these angles are called opposite interior angles.
Two figures are said to be congruent if one can be moved to another by an invertible isometry.
If the three angles in one triangle are congruent to the three angles in another triangle, the two triangles are said to be similar.
The length of a line segment is the distance between its endpoints.