In the orthographic views above, we see three views of an object with two holes in it. Holes in objects can be thought of as internal curved surfaces; they have the same characteristics as the external curved surfaces we just learned about, except for one important difference: they will show up as hidden lines in the views where we can't see their curvature. Looking at the views above, you'll notice that the top and right side views of the object's holes show that one of the holes continues all the way through the object, while the other does not.