The LSE/DECwindows interface provides pull-down menus, pop-up menus,
  and dialog boxes that allow you to issue and/or get additional information
  about menu items.

  To display the contents of a pull-down menu, position the pointer on an
  item in the menu bar and press and hold MB1.  For example, if you choose File
  in the menu bar, the File menu name is highlighted and a pull-down menu
  appears.  To choose a menu item, move the pointer down to the desired
  item.  You do not need to keep the mouse button pressed while you move
  the pointer to the desired item.

  To display the contents of a pop-up menu, position the pointer within a user
  or review buffer and press MB3.  To choose menu items, position the
  pointer on the desired item, and then press MB3.  To avoid choosing an
  item on a pop-up menu, press the MB3 mouse button outside of the menu
  borders.