I was several years into my Emacs usage before mis-types revealed the glory of the “narrow” and “widen” functions to me. Actually, it was one of those nightmares: “Whoa! Where’d my document go!? And why isn’t “undo” bringing it back?” When the tears subsided, I soon realized how useful these commands are. By selecting a region and narrowing it you focus your editing and attention on a portion of what might otherwise be massive files; by so doing, your search/replace commands and your commands to jump to buffer boundaries are contained in a useful area.