In TOS their foreheads were flat exactly like those of humans. The only distinguishing mark was their beards. In TMP and thereafter, however, the Klingons had ridges on their foreheads, to emphasize their being alien but probably also their martial nature.
Interestingly, the Interplay game "Klingon Academy" contains a dramatically different interpretation. General Chang vouchsafes that an ancient three-bladed weapon was the inspiration for the emblem, chosen by Kahless to represent the virtues he felt were most important; Duty, Honor and Loyalty.
In 2290, a cure for those Klingons who had been resistant to the earlier Augment virus-cure was finally developed. Both Kor and Kang were given this cure, and had their standard Klingon appearance restored as a result.
The changes are overall slight enough to be interpreted as a part of Worf's natural aging process. The make-up was not returned to the initial version of the first season for the flashbacks in TNG: "All Good Things" though.
Gone were the forehead bumps, beards, long locks, and otherwise human faces that, for decades, had denoted the standard for a proud Klingon warrior. In their place were macrocephalic and hairless craniums with flaring, four-nostriled noses, flat ears, glossy skin, and a Bolian-like chin line.
Riker gains the respect of the Klingons | Star Trek TNG
Why did Klingons develop ridges?
Most notably the make-up of the Klingons was modified for TMP. In TOS their foreheads were flat exactly like those of humans. The only distinguishing mark was their beards. In TMP and thereafter, however, the Klingons had ridges on their foreheads, to emphasize their being alien but probably also their martial nature.
With a greatly expanded budget for makeup and effects, the Klingons were completely redesigned for Star Trek: The Motion Picture (1979), acquiring ridged foreheads.
Although Worf was raised by humans, he considered himself a Klingon at heart and studied in-depth the ways of his people. As an adult, his mannerisms and personality, as well as his innate sense of honor, became more Klingon than human.
Worf's correct last name is Rozhenko. It's his adopted name. Klingons don't have last names. They are referred to as [name], Son of [father's name], or House of [family's House name].
Worf is an outsider to Klingon society, marked as different both by his positionally — adopted child of humans, member of Star Fleet — and by his unshakeable commitment to the values that Klingons espouse.
Memory Alpha notes that there is no canonical maximum estimate for Klingon lifespans, but several Klingons are known to have exceed a century, and some may have made it past 150 (earyh) years. That makes them pretty comparable to 24th century humans.
After season 1 of TNG wrapped production, Worf's facial prosthetics were either lost or stolen. This presented Michael Westmore, TNG's primary makeup designer, with an opportunity to improve the look of the character.
Star Trek: Lower Decks season 4, episode 9 reveals that Klingons have pink blood, a departure from their usual red blood. The choice to give Klingons pink blood was made in Star Trek VI: The Undiscovered Country to create an alien look and to avoid a higher film rating.
petaQ. One Klingon term used as an insult on numerous occasions was petaQ (also spelled "Pahtak", "Pathak", "p'tahk", "p'takh", "patahk", "pahtk", "p'tak", or "p'taq"). It was also used by the Klingons of the mirror universe.
Worf was Married to Jadzia Dax and (spoiler) at her death in DS9 became a widower. He did have a fling with Deanna Troi through 2370, which was the result of Worf seeing himself Married to Counselor Troi in an alternate reality which led him to actually pursue Deanna romantically which was welcomed by her.
In later episodes, Troi has romantic involvements with several others, including a brief relationship with Klingon Starfleet officer Lieutenant Worf. A major exploration of their relationship begins with "Parallels", in which Worf encounters parallel universes where they are married with children.
Later in the series, she becomes involved with the Klingon character Worf, and they marry during the sixth season of the show. In the sixth-season finale Jadzia dies but the Dax symbiote survives, and in the seventh-season premiere returns with a new host Ezri Dax, played by Nicole de Boer.
The baldric didn't just show that Worf was Klingon. It was also an important celebration of Klingon heritage. While other baldrics might have been used to carry weapons, the Klingon ones were solely for honoring their family or house, something that was important to Worf.
Captain Worf's protest, "That was not my fault!" is all Star Trek: Picard season 3 says about the missing USS Enterprise-E. It's a hilarious joke, and Picard showrunner Terry Matalas deliberately chose to leave the true fate of the Sovereign Class starship up to the viewer's imagination.
A small number of people are capable of conversing in Klingon. Because its vocabulary is heavily centered on Star Trek-Klingon concepts such as spacecraft or warfare, it can be hard for everyday use because of the lack of words for a casual conversation.
The official explanation is that Klingons normally have hair but completely shave it off during a war. Pronounced. The DIS make-up includes cheek prosthetics.
Later series introduced cranial ridges, and Discovery created a more alien look for the Klingons. Genetic modification is explored as a possible explanation for the appearance differences, with Enterprise featuring a storyline involving a virus that caused Klingons to lose their distinctive features.
This episode "reunites" three Klingons from Star Trek: The Original Series, one from each season: Kor (from "Errand of Mercy" in season one), Koloth (from "The Trouble with Tribbles" in season two), and Kang (from "Day of the Dove" in season three).
