What Is The Story Behind Pierrot And Harlequin?

Forgive me for my spelling, please.
i was reading a book and they made a somewhat vague reference to Pierrot and Harlequin, and I tried looking up their story but i couldn’t find anything.
I know that Pierrot was a melancholy and innocent character, and Harlequin was a tricky acrobat, but what is the relation between the two characters?
what stories are they from, and is there any more information you can give me on them?
Thank you!