Regimental Allies: Praetorian Guard
Despite being located on nearly opposite ends of the Imperium, Tyrellia Prime and Praetoria have enjoyed a long and amicable relationship, bonded by cultural similarities - While their attitudes toward the lower classes are radically different, their aristocratic norms and customs are similar enough that that their respective officer corps quickly became fast friends, bonding over the strangeness of society on other worlds. Indeed, it was a Praetorian merchant vessel that re-discovered the Tyrellia System, and their cultural compatibility made for an absolute minimum of culture shock, once the Praetorians got over their frustration at failing to snare the natives into unequal treaties.
As the two cultures warmed to each other, the Praetorian Guard became the primary influence on the modernization of Tyrellian military doctrine, from their uniform jackets and helmets to the adoption of socket bayonets and firing lines to replace the pike-and-shot warfare that dominated Tyrellian battlefields at the time.
However, the common soldiers of Tyrellian regiments rarely get along with the aristocratic Praetorian officers, and relations can become strained if a Praetorian officer makes the mistake of treating a Tyrellian fusilier the way he would a Praetorian footman. Still, the fact that their cultures have similar approaches to resolving personal conflicts peacefully or through individual duels keeps the fallout from such incidents to a minimum, and they often result in the enlisted men of both regiments bonding over their dislike of Praetorian officers.