Talk:Change of variables theorem

What is the theorem?
What is the Change of variables theorem? Currently it only redirects to Integration by substitution - where I cannot find anything about this theorem. Also under Change of variables, I cannot find anything about it. Google directed me to this site, maybe a user familiar with this topic can add clarification quickly. AS: I came here from Jeffreys prior. Greetings, --Qaswed (talk) 14:57, 4 April 2017 (UTC)