Key points are not available for this paper at this time.
An investigation is made of the periodic three-dimensional systems of points in which each point is connected to three others. These nets form the bases of the structures of a large number of crystals in which there are directed bonds or hydrogen bonds. * This condition excludes an 'appendix' connected to the main body of the network by a single link.
A. F. Wells (Mon,) studied this question.