"Brothers, I say to you the darkest secret of all. That Horus, the cursed one, did want the God-Emperor's holy hat box for himself and so that is why he did spit on his oaths and try to wrestle the box from the hats of the Emperor. Thus is why the heresy began, the greatest shopping sale cat-fight in the history of mankind."
- – Marneus Calgar reveals the dark, dark truth to the brothers of his chapter in a grand assembly.
"It was a tough call for me to make, but unfortunately that goes with the hat"
- – Ciaphas Cain HERO OF THE IMPERIUM
In the grim darkness of the far future, the size of your hat determines how important you are. If your hat appears to be larger than those around you, you may be an important person!
You do still have to kiss the ass of anyone with pauldrons, but whatchagonnado?
Consult your local Imperial administration office for further instructions if you believe you have a large hat.
Note that if your hat has frills, you may instead be a little girl. Should this be the case, please contact your local shrine maiden. If it has stars, a pointy tip, wide brim, and/or WIZZARD written on it, then you are probably a wizard or mage, and should seek help for what may likely be magically-induced memory loss.
Also, notice that helmets are not a form of hat, in fact, they are quite the opposite. While a Hat makes you more important, a helmet makes you less important. This can be simply seen in the armies of the Imperium, and indeed, of the forces of Chaos and the various Xenos as well, regular troops in squads have helmets while the squad's commander only goes out with either optics, comm equipment, hat, or Astartes-pattern Bahldness to signify their rank or duty, but never a brain cage. The only exception is that they need it in extremely hazardous environments. This shows a clear social hierarchy in head and shoulderwear:
Helmet < No helmet < Hat < Pauldron + Helmet < Just Pauldron < Pauldron + Hat
It is the sign of sheer manliness to walk out in the battlefield with just a piece of cloth material on your head. Fortunately Slaanesh, who in its spare time is the deity of fashion and looking good, smiles on those who dress well on the battlefield and therefore gives an automatic plot armour save to those with hats, meaning all the goons will have to go down first before the bullets and blades start coming in your direction.
Orkz have embraced this human custom. Eldar say that they had mighty and noble hats millions of years before the first early hominid put rocks on their heads. If one is of La or O rank in the Tau Empire can often be determined by his/her headgear. Tyranids, unfortunately, do not have the manufacturing capability to produce hats, so instead the size of your forehead spike counts in a similar way to the size of your hat in conventional armies. That is why the Swarmlord is the boss, as he has the biggest and most majestic of all the forehead spikes. Although occasionally, some Tyranids wear hats, when the mood strikes them. Even the Necrons in the wake of their latest update have taken to the habit of bolting down hats onto their metal skulls. The split between the C'tan and the Necrons was all caused by one shocking secret; that the C'tan were hording all the best hats in the universe for themselves.
Take note that no-one has ever been seen as so important as to have Pauldrons AND a Hat, not even the Empra. The Emperor's most glorious and sexy laurel leaf is his hat. Only the Classy Marines are given the utmost honor of donning a hat in tandem with their pauldrons for optimum classiness (although it should be noted that the rank and file Classy Marine has pauldrons, a hat and a helmet). Actually, Magos Dominus of the Mechanicus get to wear both, showcasing why the Mechanicus still allow the Imperium to get shit done. In addition, Imperial scholars have concluded that not only does the Emprah's laurel leaf+halo count as a hat, it counts as the best hat, thus avoiding a massive schism in the Imperial Truth.
Few people know that were GW to actually move the turgid plot of 40k along, it would be revealed all the races are in fact fighting for a cargo ship full of hats.
So that's what Abaddon really came to Cadia for; he wanted to split the planet open to get at the rich hat-veins Creed had hidden there in case he needed to be MORE manly than he already was
WHAT THIS ISN'T TEAM FORTRESS 2 GABEN BE PRAISED! POOTIS SPENCER APPROVES THIS ARTICLE!
Oh, wait. Space Marine Librarians wear psychic hoods with their power armour. Make of that what you will.
Warhammer Fantasy follows a similar style, with hats often denoting how important you are; except that instead of officer's hats n shit, it's super fucking cool Renaissance period floppy hats, which make everyone look like a Landsknecht. Akin to how in 40k, pauldrons are seen as more important than hats, in Fantasy, Mustaches are seen as a more powerful denotation of your importance. The Emperor, Karl Franz, has no mustache or hat (but likes to collect them according to Kruber from Vermintide), but makes up for this disability with common sense and Summonings of the Elector Counts. The exception to the trend is Nagash, who wears what looks like a cross between an Ancient Egyptian Pschent Crown and the pope hat in The End Times, and given that his new body is 60 feet tall, the hat is scaled to match. His right-hand man Arkhan wears a hat like a mini version of Nagash's while Neferata wears a similar hat so Arkhan-sempai will notice her.
The Chaos Dwarfs of Warhammer Fantasy at times may be appear to be the same size as humans, and this is due to how over 50% of a given one may in fact be hat. Therefore, they utterly outstrip the Empire in sheer hattery, though ironically this never denoted to Geedubs that they were important (or, obviously more vital, very profitable).
For some reason, no one wears hats in Age of Sigmar, except Nagash and some of the undead, of course Nagash has the biggest hat of them all.
Magic the Gathering now recognizes the importance of hats.