Line Weight
Lines have weight? What does that mean? Line weight is the thickness of the line relative to the other lines in the drawing. In the example below, the twelve lines making up the first box all have the same weight, but notice how each line tapers as a result of being thrown. So even though each line is similar to the other you can vary the weight across the line to give a little more interest to the drawing. This first kind of line weight is useful for constructing a drawing. Notice that even the edges of the box that wouldn't otherwise be visible have been drawn. But this line weight alone is not enough for a finished drawing. 

In the next example, the lines of the box that are visible edges have been thickened, giving them additional 
weight. The edges that are hidden are left alone. But now the line weight is a little too 'regular'. 

In the third example, the two base lines and the line up the right side have been given additional weight. This is a technique that implies a light source--in this case from the upper left. Now the box is again more 
interesting, and we have established a hierarchy of line weight. 
 
                   1.                                      2.                                         3.
[construction and hidden lines]             [visible and outlines]             [implied shadow and base lines]