Retinue of Retinues
The phrase I drill into my student’s heads about the structure of medieval armies is that they are a retinue of retinues. What I mean by this is that the way a medieval king raises his armies is that he has a bunch of military aristocrats (read: nobles) who owe him military service (they are his ‘vassals’) – his retinue. When he goes to war, the king calls on all of his vassals to show up. But each of those vassals also have their own bunch of military aristocrats who are their vassals – their retinue. And this repeats down the line, even down to an individual knight, who likely has a handful of non-nobles as his retinue (perhaps a few of his peasants, or maybe he’s hired a mercenary or two on retainer).
…The average retinue…was five men although significant lords (like earls) might have hundreds of men in their retinues (which were in turn comprised of the retinues of their own retainers). So the noble’s retinue is the combined retinues of all of his retainers, and the king’s army is the combined total of everyone’s retainer’s retainers, if that make sense. Thus: a retinue of retinues.