Exactly what makes weapons one handed or two handed, if it isn`t the weight?
It is the weight, kind of.
Okay, so the real answer would be stability and leverage. Normally, if you have the option to, you’ll want to two hand most weapons. This includes things like, “one handed,” swords.
The heavier a weapon is, the harder it will be to stabilize it with one hand. So, like I said, it is “kind of,” the weight. You may not need a second hand to operate a sword (and this includes things like the greatswords), but a second hand on the hilt will make the weapon much easier to control.
It’s important to remember that weapons (especially melee weapons) aren’t particularly heavy. When we’re talking about something like a greatsword, it’s going to weigh less than your house cats. It will weigh significantly more than a sword or bastard sword. But, if there’s one takeaway, it’s that swords, axes, and the suite of other melee weapons, are all light enough to use all day. They’re light enough to carry for miles as you travel to the battle, and light enough to kill people for hours at a time. A truly heavy weapon will wear its wielder out before they even reach the front lines. Carrying a forty pound greatsword to battle would see your soldiers arriving already fatigued, and they’d be exhausted before the battle’s first hour had passed. (In fairness, your heavy infantry would be carrying that much weight in their armor, but they’d also have extensive conditioning. And, unironically, when you do see the weight inflation of weapons in fiction, you almost always see similar degrees of inflation for their armor; So you end up with situations where your character’s weapons and armor weigh more than they do. There’s issues here.)
So, being able to hold a weapon with both hands will make it significantly more stable, and if your hands are separated by any significant distance, that will also help with leverage (which further improves stability.) These both improve precision and control.
This is part of why modern handguns are two handed weapons. They’re remarkably lightweight, usually under 2lbs. However, your off-hand will help stabilize your grip, dramatically improving your accuracy. (It also helps deal with recoil, which is an entirely separate discussion.)
There is a point with an extremely light and fast weapon, where you don’t get any significant benefit from an off hand. The examples that come to mind are knifes, foils, and rapiers. In these cases, you have a weapon that can be easily controlled by a single hand, and trying to two-hand it would only slow you down. Frequently, these kinds of weapons have grips that aren’t designed to accommodate a second hand, which further limits your ability to two-hand them.
So, it is, kind of, the weight. But, that’s almost more of a byproduct, most two handed weapons are still light enough that you could potentially use them in one hand. The important question is how well you can control it with one hand, and whether you need your second hand to stabilize and guide it.
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