View of the planes establishing the main curvatures on a <a href="//commons.wikimedia.org/wiki/Category:Surfaces" title="Category:Surfaces">minimal surface</a>
Eric Gaba (<a href="//commons.wikimedia.org/wiki/User:Sting" title="User:Sting">Sting</a>)
CC BY-SA 3.0
Based upon a drawing in a book