Line Width

Setting a line width of '0' means that a line will be rendered and printed at the thinnest possible width.  On high-resolution printers this may make the line appear very faint, particularly if the line is a light color.  If lines appear too faint when printed, increase their width.

Line Width is a 'world-space' measurement i.e. it is based on the world coordinates of the drawing.  A Line Width of 1mm means the line is 1mm thick in the drawing, but when that drawing (world-space) is converted to 'device-space', e.g. a computer monitor or printed page, it will appear to be a different width because of the scale factor applied to convert world-space to device-space.  Thus when you zoom closely into a drawing the line will render on your monitor at the appropriate (scaled) width.