In Creative Understanding, Roberto Torretti lays out a classification of possible relationships between physical theories by distinguishing four big types of relationships between two given theories considered in diachronic perspective: specialization, extension, reformulation, and substitution. Within the structuralist metatheory, this author and others also have propounded in several writings a diachronic typology of intertheoretical relations. Leaving besides differences of detail, both typologies reveal remarkable coincidences, although the structuralist typology is formally more precise than Torretti’s. However, in the last portion of (...) this article, I discuss a further type of intertheoretical relation in diachronic perspective, which, until recently, had not been considered by any approach in philosophy of science: what I call “crystallization” and is exemplified by the development of thermodynamics in the 19th century. (shrink)