A set of four empirical rules proposed by William Zachariasen (1906–79) in 1932, in order to explain which oxides are likely to form glasses. The rules are:

1. (1) each oxygen atom is linked to no more than two cations;

2. (2) the coordination number of cations is small;

3. (3) in polyhedra containing oxygen, corners are shared but not edges or faces;

4. (4) the polyhedra are linked to form a continuous random network. The rules have subsequently been modified, extended, and justified using topology.