In trying to understand "um euretwillen" I came across "um deinetwillen" meaning for your sake and also "um meinetwillen", for my sake. Can anyone explain this expression. I can see how "um" could mean "for the sake of" but what's got me confused is the extra "t"? Can someone help me understand?
Thanks @anion and others too. Your help gave me the terminology to search with. Here's my simple understanding: the linking together of German words sometimes involves "linking letters", and although "s" is the most common, there are others as detailed in the Fugenlaut link above. This really helped make sense of some vocabulary I've learned, and I'm sure will continue to help me.
For others sake (um ihretwillen?), I'm including "linking" in my title & tag, and for beginners like me, here is a good place to start: https://blogs.transparent.com/german/compound-words-das-fugen-s-im-deutschen-the-linking-s-in-german-part-1/
(change the last 1 in the link to see part 2)