Do you mean period is a span of time? Based on what you have said the best word I can think of is “complement”.

What is: something

• that fills up, completes, or makes perfect one
• of two mutually completing parts : counterpart See

the full definition here: https://www.merriam-webster.com.

com/dictionary/complement.

There are many ways to learn dictionary (like adds and removes), so start with a simple sentence.