The law of constancy of interfacial angles is one of the fundamental laws of crystallography. It states that for a given crystalline substance, the angles between corresponding faces are always constant, irrespective of the size, shape, or origin of the crystal, provided the temperature remains the same. In other words, crystals of the same substance may differ considerably in their external appearance and dimensions, but the angles between equivalent crystal faces remain fixed. This law is also known as the law of constancy of angles or Steno’s law, after the Danish scientist Nicolaus Steno, who first recognized this regularity in crystals of quartz. Steno observed that although quartz crystals could have different sizes and the individual faces could vary in development, the angles between corresponding faces remained the same. This observation became an important foundation for the scientific study of crystals. A crystal is a solid substance in which the constituent atoms, ions, or molecules are arranged in a definite and regular three-dimensional pattern. This orderly internal arrangement gives rise to characteristic external faces, edges, and angles. The external form of a crystal is therefore closely related to its internal structure. Since the arrangement of particles in a particular crystalline substance is definite, the orientation of its corresponding crystal faces is also definite. Consequently, the angle between equivalent faces remains constant. For example, consider crystals of quartz (SiO₂). Quartz crystals may occur as small or large specimens and may show differences in the relative development of their faces. Nevertheless, the angle between corresponding faces of quartz remains approximately 60° for the appropriate corresponding faces. Similarly, crystals of common salt, calcite, sulphur, and other crystalline substances exhibit characteristic interfacial angles. These angles are independent of the absolute dimensions of the crystals. The constancy of interfacial angles can be understood from the internal arrangement of particles. A crystal consists of a regular repetition of a basic structural unit. The planes passing through these regularly arranged particles have definite orientations with respect to one another. Crystal faces develop along such planes, and therefore the angle between two corresponding faces is determined by the internal crystal lattice. Changing the size of the crystal does not alter the lattice arrangement. Thus, the interfacial angle remains unchanged. The law does not mean that every angle observed on a crystal is the same. Different types of faces may make different angles with one another. The important point is that corresponding faces of crystals belonging to the same crystalline substance make the same angle with one another. For instance, if two quartz crystals have the same crystal structure but differ in size or degree of development of their faces, the corresponding interfacial angles will remain identical. The law can