skeleton key

noun4

1. •
   A key that has parts filed away so that it will open a range of locks.
2. •
   A master key.
3. •
   A very simple design of key that usually has a cylindrical shaft (sometimes called a "shank") and a single, minimal flat, rectangular tooth or "bit".
4. •
   (figurative) A key (A crucial step or requirement) or (An idea, theory or observation that explains, justifies or renders (something) comprehensible).

   • The skeleton key to this plan is the marketing, which will make the plan make sense when you see it.