Leader Forms the Head

"Y'all can do the treading

Swing energy machete

If combination’s ready,

I'll form the head!"

- MC Frontalot (Feat. Dr. Awkward and Zealous 1): "I'll Form The Head"

Combining Mecha tend to combine in a humanoid form. It's easier for people to follow, and it's easier for the guys to get fitting rubber suits. When mecha combine, the parts will more or less match up with limbs, and it doesn't really matter what goes where.

Except when it does.

The "face" and/or torso are special parts of the combined form. They're the most identifiable, and they're generally where the cockpit is. It's why we have special importance for the locations of our brain and heart. So with mecha, the part that is most important will be formed by the personal mecha of the most important character, generally The Hero, or a spotlight stealing variety of Sixth Ranger. Because the position is special but can only be handed to one, expect Headbutting Heroes or Vitriolic Best Buds to compete for the lead position.

At this trope's strongest, the mecha of The Hero can be humanoid on its own, and the others will form a larger body by way of a Mecha Expansion Pack, thus an overlap with Meta Mecha. At its strangest, The Hero doesn't form the head, but it gets double subverted by having the part that does look suspiciously matching to the mecha of The Hero.

Thanks to the Law of Chromatic Superiority, this leads to a lot of ensemble mechas being a bit red in the face. Also, compare the color patterns on an All Your Colors Combined attack. The main character's color illogically in the middle? This will probably be in effect when the robots show up.

Anime And Manga

 * This trope dates back to Getter Robo, the first Combining Mecha, where each of the Getter Machines' combined forms were controlled primarily by the pilot of the upper body. Getter-1, the form belonging to the leader of the group, was the most commonly used form.
 * Voltron codified this for USA audiences along with many other Combining Mecha tropes; its Transformation Sequence (played, of course, Once an Episode) comes with a voice-over of the team leader ordering each step of the process, ending with "...AND I'LL FORM THE HEAD!"
 * Played with in Voltron Force, where Voltron has different forms depending on which lion forms the torso and head.
 * Played with in Gurren Lagann. Supporting Leader Kamina makes Simon's Ganmen the (second) head. All along, Kamina had been setting Simon up to be a true hero. However, as Kamina was still the leader at this point, his ganmen forms everything but the head of the combined ganmen, has its own head on its torso, and the combined ganmen has his trademark sunglasses on both faces.

Simon, The Hero, takes the head position in his mini-mech, while the torso is taken by his Lancer, whoever that is at the time. In larger combinations, when Lagann takes over a larger mech such as the Dai-Gunzen or the Cathedral Terra, his original mech is always located within the head of the resulting gigantic mech.
 * The Big O episode 18 "The Greatest Villain". When Beck and his henchmen form their giant mecha RX3, the part Beck is controlling becomes the head. Watch it here.
 * One of the few series that averts this is The Brave Express Might Gaine, where the title mech is mostly formed from the remote-controlled supertrain Locomizer while the Kid Hero's vehicle and his transforming Robot Buddy form the arms.
 * Digimon goes all over the place with this trope in its Fusion Dances - Omegamon, the first identifiable example, has a unique head unrelated to its componenents, as does Susanoomon. On the opposite end of the spectrum, Imperialdramon's head is pretty distinctly based on that of V-mon, the Bond Creature of The Hero; its contemporaries Silphymon and Shakkoumon are both of the unique head variety. The big Combining Mecha example, Digimon Xros Wars, is also all over the place - on most of Shoutmon's Combining Mecha forms, the head is a unique one which sometimes comes from Shoutmon but is not obviously connected to him design-wise; on others, like Shoutmon DX, he is distinctly the head.
 * It is however consistent that Shoutmon's V-shaped head crest always becomes the chest piece of the Shoutmon X series, and even during the DigiXros sequences we can see his head "merging" into the chest to form that V crest.

Comics

 * Parodied in a back-up feature in Squee. The series creator, Johanen Vasquez, fought beligerent fans. The fans joined together in a grotesque humanoid version of this trope with the leader forming the ass instead of the head.

Live Action TV

 * All over the place in Super Sentai and Power Rangers, where the Red Ranger is usually the one to get the head- or torso-forming privileges.
 * Mighty Morphin Power Rangers Season 2/Gosei Sentai Dairanger and Tokumei Sentai Gobusters play the usual formations a little differently; rather than forming the torso and head Red gets his own humanoid robot and the other Rangers' mecha just form armor elements for it.
 * Mahou Sentai Magiranger/Power Rangers Mystic Force works much the same way, except that it is the Green Ranger's mecha that serves as the core that the other Rangers form armor around. However, it's still a straight example as the Red Ranger's mecha gets prime placement, slotting into the center of the chest; and most of the chestplate is red-colored as well.
 * Averted in the third season of Mighty Morphin Power Rangers, when the Red Ranger formed neither the head or torso of the Ninja Megazord. Instead the Pink Ranger was the head and the Yellow Ranger was the torso. (This was a straight example with its counterpart Kakure Daishogun Ninja Sentai Kakuranger, where the Pink Ranger's counterpart was the official leader.) Played straight in both Power Rangers and Kakuranger by the Shogun Megazord/Muteki Shogun, the team's other mecha, where the Red zord did form the core (as even when he's not the leader, Red's still The Hero).
 * In Power Rangers ZEO/Chouriki Sentai Ohranger: almost the entire Megazord is either blue, black or gold, as the Blue, Pink, and Yellow Rangers' zords are mostly blue with gold accents. However, the Red Ranger's zord, which is only visible from the back and looks ridiculously out of place, makes up the head and the cockpit. The Zeo Megazord had interchangeable helmets. The Red Phoenix helmet would usually be on top, but this trope would be invoked again when another team member had a feature episode, by swapping out with their helmet to finish off the enemy.
 * Played with in Power Rangers Time Force/Mirai Sentai Timeranger: While Red does form the head and body of the megazord, it's not at all obvious. It has two formations color-coded Red Mode and Blue Mode, and the combinations show those colors rather than the colors of the individual Rangers.
 * In Power Rangers Wild Force/Hyakujuu Sentai Gaoranger, mecha had a standardized combination scheme; some were torsos, some were arms, and some were legs; and all could be switched at will. Aside from the Sixth Ranger's mecha, all three "torso" pieces belonged to the Red Ranger. In one case, a centaur-like megazord was formed using one of the Red Ranger's zords as the head + torso and an enlarged version of another as the legs.
 * In Power Rangers Lightspeed Rescue/Rescue Sentai Go Go Five, Red forms the body and arms while Green forms the back and head... but the head is still red, and in the combined mecha it looks like part of the Red zord.
 * In Tomica Hero Rescue Force, the heroes gain a combining mecha about halfway into the series. Rescue Striker, the fire truck belonging to R1 turns into the torso and head, while Rescue Saver, belonging to R2 turns into the limbs. It is even made into a minor plot point that R2's cockpit is located inside the mech's foot.

Western Animation

 * Megas XLR
 * As expected, the show parodies this. The Glorft's UMD is a combining mecha made from other combining mecha, repeated for a couple of cycles. At its final stage, It is completed by Gorrath's personal mech docking in the center. Without Gorrath's part, it doesn't seem to be useable at all, enforced when he leaves the battlefield, and the dozens to hundreds of mechs and pilots that also made up the UMD. Gorrath having a Never My Fault mentality, all of this to be expected.
 * In Sentai parody episode "The Bad Guy", the "S-Force Super Ultra Dimensional Magno Extreme Robotoid Power Zorp" is equal parts Voltron (combining) and Super Sentai (theme), with the Red Falcon at the head. Later, Coop literally forms a new mecha by sliding/smashing Megas' limbs into their robots and slapping the bird on his back. When in fights, it becomes noticeable how uncomfortable it would be to be in a limb. The actual episode's bad guy finds this treatment disrespectful to his nemeses, and of course loves it.
 * In The Venture Bros., when JJ Venture's team spoofed Voltron with the "Ventronic." Complete with the "I form the " exclamations. The combined form had two heads, the one formed from JJ's robot, and the "left arm" clown head, shaped in a way to be recognizable and comfortable to their mentally handicapped friend Ned. Instead of just putting the clown head on top, they wound up with a stubby, useless arm, not too dissimilar to JJ himself.

Other Media

 * Transformers does this across all of its media.
 * Actually averted with The Constructicons, the First of the Combiners. Scrapper, the Leader Front End Loader of the six warriors actually forms the Right Leg of Devastator, whereas it is actually Hook the crane who forms the Head instead.
 * In the Unicron Trilogy Powerlinx has whichever Transformer is on top be in control.
 * Beast Wars Magnaboss has Prowl, the lion (whole bunch of leader archetype imagery there) form the face and center of the torso, but pretty much every other visible part comes from Ironhide to compensate for that. This results in Ironhide being rather confusingly built in his standalone form.
 * In the comics, G1's Dinobots force a combiner out of themselves, Beast, which is intentionally poorly designed, with everyone else just grafted on to Grimlock's beast mode form.
 * Partially averted in the "Scramble City type" Combiners, where the hands, feet and head may come from extra parts in the toy (out of nowhere in the cartoon). The leader of most teams is the torso. Limbs are thus completely interchangeable, between arrangement and even whole transformers (Scramble Power). In some cases, the leader of a team may actually be in one of those limbs, and thus the combined form could be configured without them.
 * Parodied by MC Frontalot in a song entitled, naturally, "I'll Form The Head". Turquoise and "Lightish Red" argue why they should be the ones to form the head, Pink because he's the most qualified and Turquoise for -- other reasons. Gold actually offers them turns, as long as they get the job done.
 * In the picture book Swimmy by Leo Lionni, Swimmy is a small fish who convinces his school to swim around in the shape of a much bigger fish to ward off predators. Being a different color from the rest of the school, he plays the eye.
 * In Bionicle, of the two known Toa Kaita combo-configurations (Akamai and Wairuha, both made up of three individual Toa), Akamai's Kanohi mask is that of Tahu, the leader of the group (Tahu's true head actually ends up on his hands). Wairuha's mask, however, is Lewa's, who is just about the bottom of the command chain, but his Kanohi was the most generic-looking, thus the most suitable for a combo model. As a subversion, in the storyline, these aren't really the masks of Tahu and Lewa specifically, nor are they a Kanohi Hau and a Kanohi Miru in general, but other types of masks that happen to look like a Hau and a Miru -- out of sheer necessity, as the designers only had these pieces to work with. They're actually called Kanohi Aki and Rua, respectively. But as a Double Subversion (for Wairuha, at least), a special edition chromed Rua was at one point made available for purchase... and it used the mold of a Kanohi Hau, Tahu's mask type!
 * The Turaga Nui figure also has Vakama's head for its real head (the other five are built into various parts of its body). Vakama is often considered the "main" Turaga of the initial six.