Two straight lines are parallel and have equal internal angles.

∵BC=CD∴∠CDB=∠CBD

Equilateral equilateral angle

Also: the vertical BD∴ triangle ADB is a right triangle.

E is the midpoint of AB, and ∴DE is the center line of the hypotenuse of a right triangle ∴AE=EB=DE.

In a right triangle, the center line of the hypotenuse is equal to half of the hypotenuse.

∵DE=EB∴∠EDB=∠EBD

Equilateral equilateral angle

∴∠CBD=∠BDE∴CB∥DE

Internal dislocation angles are equal and two straight lines are parallel.

Then: the quadrilateral CDEB is a parallelogram.

∵CD=CB∴ The parallelogram CDEB is also prismatic.

A pair of parallelograms with equal adjacent sides are prismatic.