The second phenotype was tall and thin in build, with both a single crested ridge extending over their partially bald heads and an occipital bun on the rear of their skulls (some refer to these Klingons as either specially bred "Battle-Class" Klingons, or derisively as "Duck's ass" Klingons, based on their haircuts).
The third Klingon phenotype was represented by General Chang. This Klingon was only slightly ridged in the center of the forehead, lacking the "normal" side ridges. They were largely hairless except for a small wisp at the back of the skull and (in males) a small goatee. These Klingons were seen in Star Trek: The Motion Picture.
The fourth, and most common phenotype seen, are the triple-ridged or "Worf"-type. These Klingons have individualized or clan-based ridge patterns, ranging from the slight "webbed" ridge pattern of Colonel Worf to the craggy triple ridges of the Emperor Kahless (clone).