Experts and BHG readers answer.
Thanks for taking the time to write! 20-inch spacing basically means the plant gets 20 inches wide, so you'll want to plant it at least 20 inches from the next peony. You would measure 20 inches or so from the center of your peony to the center of the next one because each peony should grow about 10 inches wide from the center of the plant to the edge. Two 10-inch segments add up to 20 inches apart.
I hope this helps!
---Justin, Senior Garden Editor, BHG.com