I don't believe I've seen this discussed before, but this question has plagued me for some time.

Under what circumstances were longswords preferred to arming swords (and visa versa)?

Given that the two types overlap historically, what drove a knight to select one over the other for any particular combat encounter? Obviously the longsword saw increased use once the development of plate rendered the shield redundant, but the arming sword seems to have stuck around even after this period.

Was it choice based on pure preference? Were there regional fashions for one sword type over the other? Were there periods of time where one type was preferred to the other?

Was the choice situational? Was the arming sword preferred for mounted combat? Most (but not all) period depictions (that I've seen) show mounted knights favoring the shorter arming sword. Was the longsword preferred for infantry combat?

Was the choice dependent on other equipment choices? Do longswords become sure favorites when we control for a certain level of armour coverage? I know the Italian and Spanish Rondelleros, lightly armoured as they were, preferred sword and buckler to longsword, but was this the case with knights?

Does combat role have any effect? Did troops with certain specific roles on the battlefield (other than the rondelleros) exhibit strong preference?

Hmm...I recall a discussion not long ago about the tendency among mounted men-at-arms to carry both a longsword and an arming-sword-one on the saddle, one on the belt. Try searching about that subject in the older threads.

I posted ir. Get a copy of Chronicles of the Crusades, and read The Life of St Louis by the Seur de Joinville. He tells how his life was saved by carrying an arming sword on his belt . He got ambushed in such close quarters that he couldn't draw his longsword, which hung on his saddle because of it's length. He cut his way out using the arming sword.This was the whole point of the arming sword, at 22'to 26" it was easy to carry, but was wide enough and pointed enough for regular combar against both textile and steel armour and for flat-parrying a regular size sword,plus of course people wore so many layers of wool and linen in winter they might as well wear textile armour. The arming swords shown in Records of the Medieval Sword are mostly downsized type XII,typeXIV or typeXVIII. They were also called riding swords as they were generally carried whenever the owner went out the door. Their basic use was up close intense personal combat, or for when the owner was climbing walls during a seige or assault, or fighting on board a ship