YDS (USA), EU, FR, Alaskan, V-Scale/Hueco (USA), B-System, Fontainebleau, Dankyu, UIAA, British Trad…

Lots of climbing websites have this information, luckily the website, The Crag, has collected everything into one place! They have a handy tool that converts grades for you:

https://www.thecrag.com/en/article/grades

So now we’re here to summarize the history behind these different systems, coming soon!

Others:

http://www.climbingtechniques.org/understanding-route-grades.html

https://www.planetmountain.com/news/2004/03/boulderinggrades.html

http://www.rockclimber.de/index.php?Itemid=57&id=54&option=com_content&view=article

https://www.mountainproject.com/international-climbing-grades