Help > Authoring > Equations > Using the Equation Editor > Using the Mouse in the Equation Editor > Middle Mouse Button
  
Middle Mouse Button
You can use the MIDDLE mouse button (LEFT+RIGHT on a two button mouse) for a variety of functions. For example:
Click the MIDDLE mouse button to highlight the symbols between the outlined symbol or construct and the current position of the pointer.
Click once on each character with the MIDDLE mouse button to highlight symbols one at a time.
Click the MIDDLE mouse button at either end to extend or reduce the highlighted region.
Hold down the MIDDLE mouse button while dragging to extend or reduce highlighting.
In the Palette, hold down the MIDDLE mouse button to drag Palette buttons from one place to another on the Palette.