Draw a line under or between something

To draw a line under something is to close something off or end something. To draw a line between things is to separate and distinguish between them. Both metaphors derive from writing/accounting and date from the 18th century, as does draw the line, which means to define a limit beyond which one will not go. Lines drawn across pages of writing or accounts generally separate one subject matter from another and their use in a figurative sense derives from this.