Its function is to provide you with a "testing" background image for websites that are to be no wider than 640 pixels or less.

This background is left justified and has a white color for 640 pixels. To the right of this width the background appears black. If you use the "right edge anchor" (the double headed arrow indicator) to move the right edge of the window further to the right, then the right side of the window remains black in color.

To capture this background image press the right mouse button, then save the background image with the appropriate selection provided in the drop down box.

The image currently is called "640bg.gif" . When viewed with the browser it will appear as a single line of white with a black line starting at the 640th pixel.

The table border shown here is that of a <table> block whose width is set to 600 pixels (width=600).