In geometry, parallel lines are lines that never meet, no matter how far they are extended. Conversely, perpendicular lines are lines that intersect at a 90-degree angle. Both of these concepts are important in understanding geometric figures and their properties.

Congruent triangles are triangles that have the same size and shape. This means that all corresponding sides and angles of the triangles are equal. Congruence can be shown by using a series of transformations, such as rotations, translations, and reflections, to match one triangle to the other.