Steno's Law

the law of the constancy of interfacial angles in crystals. Steno’s law states that in all crystals of a given substance at a given temperature and pressure, the angles between corresponding faces of the crystals, regardless of the size and shape of the faces, are always the same. The law was established by N. Steno in 1669 on the basis of observations of the varied shapes of natural crystals and is explained by the correspondence of the faces of a crystal polyhedron to flat networks of atoms in the crystal lattice. Steno’s law forms the basis for the classification and identification of crystalline substances. The angles are measured with a goniometer.