Definition:Complements of Parallelograms
From ProofWiki
Let $ABDC$ and $EFHG$ be two parallelograms with the same angles, which share a diagonal.
Then the two parallelograms $CIGK$ and $BJHL$ are known as the complements of the parallelograms $ABDC$ and $EFHG$.
Linguistic Note
The word complement comes from the idea of complete-ment, it being the thing needed to complete something else.
In this context, the thing being completed is the large parallelogram $AIJF$.
It is a common mistake to confuse the words complement and compliment. Usually the latter is mistakenly used when the former is meant.
