Quote Originally Posted by llywellyn View Post
What are the core parts of a map that must always be included (e.g., compass, scale)?
Well, what you include depends on what type of map it is. You certainly would not always include a compass rose or a scale because not all maps will preserve compass bearings or scale. This is particularly true of world or other small scale (i.e., large area) maps, due to projection issues, but may also apply to much more local maps. A typical subway system map will be laid out for easy of use and distort both distance and direction to simplify and maximize readability.

There are elements which are fairly common, such as borders or a key, but I can't really think of anything that is mandatory.