With a bit of imagination, we can visualize some planes in the face: Let's take a look at this store mannequin as a slightly simplified example. The human face is a favorite subject and it has many complex and subtle changes of plane. Some are sharp and some are very gradual. Most of the time, our subjects are much more complicated, with many different changes of plane.
So far we've looked at very simple objects with quite basic changes of plane. The die on the right is drawn this way, with broken lines suggesting the more subtle curved edges. This can help you create the effect of the gradual change of plane. The brain interprets these broken lines as being less sharp or hard than the solid lines. If varied line weight is being used, we can lift the pencil off and then on again gradually, or we can use a clean break or a dotted line. An implied line uses a slight break in the line to suggest that an edge is there, but it isn't as strong as other lines in the drawing. The other option is to draw using implied line. However, it does make the edge look much harder than it really is.
Sometimes this can work quite well and the somewhat visible edges on the face of the dice mean you can get away with a solid line in this case. We can also draw as close to the edge of each plane as we can, leaving the curved area between them. Sometimes we can make a 'best guess' as to where the middle of the change of plane is. But what about the edges between two planes facing us? They form a gradual curve. When the change of plane happens against the background, it's easy - that outline is clear and sharp. The change of plane happens more gradually and it isn't at all crisp. Here are two more boxes of sorts, but there's a complication: the edges are rounded. Now that we've looked at a box with nice crisp edges making a clear change of plane.