What is 'Weight of Line'?...

Adding line depth (how hard you press) to your sketches will result in the lines becoming a little thicker and darker, or a little thinner and lighter. This technique makes your sketch look more 'dynamic' - has movement and excitement, as you highlight some areas for the viewer. It is much easier to do using a mechanical pencil with blue lead as the pencil end is always sharp. This means that you can have a very dark blue line when you press hard or a very light sharp blue line when you press lightly.

As a simple rule to follow (although rules can be broken of course!) if a line has a concave curve shape then it would get darker on the line i.e. a cave is always dark inside. You should always also darken where lines meet, to communicate detail in your sketch.