the book. One of them, the term "crystal", for which crystallography had an un­ ambiguous definition, is now employed by biologists to describe the structure of cell membranes and by chemists who use it to denote degrees of polymer crystallinity. "Crystal" has thus become a broad term that can help describe any solid, or just a condensed state of a substance, if the solid has a suf­ ficient degree of order in the arrangement of its components. But the book is called "~lixed Crystals". The other word in its title, the adjective "mixed", has also developed several meanings. It is now thought ap­ plicable to both homogeneous and heterogeneous systems, that is, to crystals composed of different molecules and also to solids that are a mixture of crys­ tals with different structures.