a unary phase diagram
Take the simplest possible case: a diagram for a single pure substance, one component only. With composition frozen (it is always 100 percent of that one thing), the only two knobs left are temperature and pressure. So a unary diagram is a map with pressure up the side and temperature across the bottom, carved into regions that say which single form is stable where. The water version — with its solid, liquid and vapour fields meeting at a triple point — is the one everyone has seen.
The ceramist's flagship unary diagram is pure silica, SiO2. Heat it at one atmosphere and it marches through a sequence of solid forms (polymorphs) before it melts: alpha-quartz up to 573 degrees C, then beta-quartz, then tridymite from about 870 degrees C, then cristobalite from about 1470 degrees C, and finally liquid at about 1723 degrees C. Each boundary line is a temperature at which one crystal form gives way to the next. The phase rule bites here: for one component at fixed pressure, a single phase has F = 1 (you can change temperature freely), but where two solid forms coexist F = 0, so each transformation happens at one fixed temperature.
This is not just trivia. The quartz inversions carry volume changes that crack ceramics: the alpha-to-beta quartz flip at 573 degrees C jumps about 0.8 percent in volume almost instantly, which is why a body full of quartz must be heated and cooled gently through that temperature or it will craze. The tridymite and cristobalite forms matter in silica refractories and in why fired whiteware behaves as it does. A unary diagram is also the honest reminder that temperature alone, at fixed composition, already decides which structure you have.
Fire a silica brick and cool it slowly. On the SiO2 unary diagram you cross from liquid to cristobalite near 1723 degrees C, then in principle toward tridymite and quartz on the way down. In practice the transformations are sluggish, so a silica refractory ends up a mix of cristobalite and tridymite frozen in — the diagram tells you what is stable, not how fast it gets there.
One component, axes of temperature and pressure; the silica diagram maps quartz, tridymite, cristobalite and liquid.
A unary diagram shows equilibrium, but silica's polymorph changes are so sluggish that non-equilibrium forms routinely survive to room temperature — cristobalite is often found in fired ware where quartz would be the true stable phase.