The midpoints of the sides of an arbitrary quadrilateral form a parallelogram. A parallelogram with equal diagonals is a rectangle. The diagonals of a parallelogram bisect each other. In particular, the centroid of a parallelogram is the meeting point of its two diagonals. A parallelepiped can be considered as an oblique prism with a parallelogram as base. The three-dimensional counterpart of a parallelogram is a parallelepiped. They form a rectangular prism with a parallelogram as its base. Vectors are added using the parallelogram law or the triangle law. Any line through the midpoint of a parallelogram bisects the area and the perimeter. Any line through the midpoint of a parallelogram bisects the area. The curvature arises when one travels around an infinitesimal parallelogram. The area of the fundamental parallelogram of the lattice is formula 82. Exact analogs of the Pythagorean theorem and parallelogram law hold in a Hilbert space. Without further symmetry, this parallelogram is a fundamental parallelogram. It can also be defined as a parallelogram containing a right angle.