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"If two vector quantities are represented by two adjacent sides or a parallelogram then the diagonal of parallelogram will be equal to the resultant of these two vectors."

The parallelogram law of vectors states that if two vectors are represented by the sides of a parallelogram, then the diagonal of the parallelogram passing through the point of intersection of the two vectors represents the resultant vector. This means that the sum of the two vectors is equivalent to the diagonal vector.

The parallelogram law of forces says that the sum of two forces is equivalent to the parallelogram formed by placing the first vector as starting from the origin and the second starting from the head of the first. This can be proven through trigonometric derivation of triangle angles and sides.

How it is related is given in the following link:

http://blog.oureducation.in/to-verify-the-law-of-parallelogram-of-forces/