FB Mutual Friends:  I am friends with both Jack & Jill and they are friends with each other.  I have my Friends list set to only me.  When I view my timeline as View As-Jack, I see mutual friends, including Jill. When I view my timeline as View As-Jill I see Jack as a mutual friend.  I know Jill has her friends list hidden (at least to me), so this would mean that Jill has her list visible to Jack?  Or is Jack able to see Jill as a mutal friend (when viewing my timeline) because his list is visible?  Trying to understand the mechanics.  
Who's friends list has to be visible to see FB mutual friends?
facebook;friend list