The midline theorem, formally known as Varignon's theorem, states that a parallelogram is formed when the midpoints of the sides of any convex quadrilateral are connected in order. The area of the Varignon parallelogram is half of the original quadrilateral.
If the two diagonals of the quadrilateral are equal in length, then the Varignon parallelogram is a rhombus. If the two diagonals of the quadrilateral are perpendicular, then the Varignon parallelogram is a rectangle.
Varignon's theorem was published in 1731 and is part of Euclidean geometry. The field of mechanics has its own theorem by Varignon. These theorems are similar in name, but they are completely different in function.