Selection bar
 
An invisible area (two characters wide) just
inside the left window border, used with the
mouse to select a line, a paragraph, or the
entire document.
 
To make selections, move the cursor into the
selection bar with the mouse:
■ To select a line, click the left mouse button.
■ To select a paragraph, click the right mouse
  button.
■ To select the entire document, click both mouse
  buttons at the same time.