Complex sentence: A third type of ruled surface is the hyperbolic paraboloid; it has, again, two families of skew lines; in each of the two families the lines are parallel to a common plane although not to each other.
Split into simpler sentences: A third type of ruled surface is the hyperbolic paraboloid. Like the hyperboloid of one sheet, the hyperbolic paraboloid has two families of skew lines; in each of the two families the lines are parallel to a common plane although not to each other.