A selection bar is an invisible narrow strip that runs along the left edge of a Microsoft Word document window, according to The University of Texas at Austin. It is used when selecting a line of text, a paragraph or an entire document.
When the mouse pointer is placed in the selection bar, it turns into a right-pointing arrow. Clicking the left mouse button while the pointer is in the selection bar selects the line to the right of the pointer. Multiple lines can be selected by holding down the left mouse button and dragging the pointer. Double clicking in the selection bar selects the entire paragraph, while triple clicking selects the entire document.