Step 1: Coal forms from buried plant matter that slowly changes under heat and pressure over millions of years. Step 2: The first stage is peat (about 50–60% carbon), which is the least developed. Step 3: With more time and pressure it becomes lignite (about 60–70%), then bituminous coal (about 70–85%), and finally anthracite (about 90% or more). Step 4: More carbon means higher heating value, so anthracite is the best quality coal. Answer: Peat → Lignite → Bituminous → Anthracite.