Explainer: What is the purpose of the U.S. presidential nominating conventions?

FILE PHOTO: Balloons drop at the conclusion of the Democratic National Convention in Philadelphia, Pennsylvania, U.S. July 28, 2016. REUTERS/Scott Audette

WASHINGTON (Reuters) - The United States has held presidential nominating conventions for almost 200 years, and they have served in recent decades as an important televised introduction to each party's candidate in the final months before the election.

Here is a look at the upcoming Democratic and Republican national conventions, which kick off on Monday and run back-to-back weeks. The presidential election is Nov. 3.