Separator

--- alone on a line (anywhere except the first line of the file) draws a line across the screen.

Above the line.

---

Below the line.

Preview: Commodore 64, 40 columns

                                            
  Above the line.                           
                                            
  ────────────────────────────────────────  
                                            
  Below the line.                           
                                            

Put a blank line before it: in Markdown, a --- directly under a line of text turns that text into a heading. The separator is styled by the stylesheet key rule.