Definition:Complements of Parallelograms

Let $$ABDC$$ and $$EFHG$$ be two parallelograms with the same angles, which share a diagonal.


 * ComplementsOfParallelograms.png

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.