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I've gotten interested in the metallurgy of blades, and been watching a lot of early Forged in Fire. It's interesting to see basic applications with different blades by people who handle a lot of blades. There are fundamental similarities between blades of all types with the same primary functions, for instance, many of the knives and swords the smiths are challenged to create are used for hacking/chopping.

Length is the obvious answer, but here I'm asking about the physics of application—how are knife strikes mechanically different from sword strikes.

And length alone may not be the determining factor because a kris (dagger) can be up to 50 centimeters in length, which is within range of the Roman gladius and Ancient Greek xiphos (sword). The Spartan xiphos had even shorter blades of 30 cm (12 in), analogous to a long bowie knife.

Weight may be a factor, but not all the sources I'm using list blade weights, and this factor seems also omitted from FOF.

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    I suspect the difference might be in the intended purpose. Which would make machetes knives. But then a dagger would be a sword. Interesting question!
    – Vorac
    Jul 19 at 12:41
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    @Vorac is correct, utility/purpose has more to do with the distinction than size does. See katana/wakizashi/tanto and the variants (tachi, etc)
    – JohnP
    Jul 27 at 16:53
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    @JohnP I personally think it has to do with the ability to snap a cut by using some point on the blade as a lever. But that's just my guess, based on experience. With a knife, the power all comes from the body. Certain sword chops are similar. But with a proper sword, you can also generate much greater momentum and force via the whip-like snap that is the core of good cuts.
    – DukeZhou
    Aug 10 at 21:54
  • @DukeZhou - I will defer. I have limited practical experience with sword.
    – JohnP
    Aug 11 at 14:36
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It's dependent on context. Sure, knifes are smaller, swords are bigger. A clear line does not exist. A bronze age sword can be a puny thing compared to a Grosses Messer.

In using it as a weapon it depends on the characteristics of the weapon and the user. The way I was trained (Philipino martial arts) knifes are used more for slicing draw-cuts and repeated punctures. There is simply not enough mass to chop with it.

With a Long Sword you can stab, slice, chop. Of course you'll be in a different range and may want to keep it this way.

Machetes can be quite different based on the origin. Longer, thinner blades where the vegetation is lighter. Or heavy choppers. The Phillipines also are home to machete-sized weapons that are designed for war or head hunting.

There are many cultures with different traditions and many blades. You'll handle a 3 cm blade just very different than a 7 inch bowie or a cutlass or a arming sword.

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  • I agree that the difference is largely semantic. In some districts, it may also run into legal distinctions, such as laws that distinguish between "knives" and "daggers" in terms of legality of carrying one. I'd personally put machetes in the category of swords, in part because of the chopping nature of using one, versus knives which tend to be more about stabbing or slashing, not that you can't do those with swords as well. Jul 27 at 14:21
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"Knife" and "sword" exist on a spectrum, with some things definitely falling on the "knife" end, some definitely on the "sword" end. This means that there's no hard criteria placing things in one category or another.

Historically, there seems to also been some legal differences between them, in some areas. It is claimed that one of the reasons for the German "lange/grosse messer" being that swords were limited either by what guild(s) could make them, or who were allowed to wear swords, because the main difference between a falchion (definitely a sword) and a langmesser is that the falchion tends to have a hidden tang and the messer a full tang.

So, in short, there are things that are definitely knives, there are things that are definitely swords, and there are things where you can have an argument what it is.

If we separate edge weapon attacks into "stab", "cut" and "slice", a sword is more towards the "stab" and "cut", whereas a knife is more towards "stab" and "slice", but even that is more of a tendency than a hard distinguishing criterion.

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This is just based on ~30 years of research, swinging swords of all weights, with some training. I practiced cutting on objects extensively when I was starting out.

  • What distinguishes a sword is the ability to "snap" a cut

What is meant here is using the weight and balance of the sword itself to create a force multiplier, utilizing an axis point somewhere along the blade as a fulcrum or "lever" or "pivot". The focus of this kind of cut is analogous to the physics of cracking a whip—movement in one direction, then oppositional force to generate maximum power.

Knife strikes use the force of the body only. A knife is too short to have that pivot, and daggers are too light. If a knife is long or heavy enough to snap cuts, it is technically a sword. A machete is a useful tool for cutting brush because of that snap, which leverages the relatively light weight. Bowie knives tend to have tapering blades, so even with the high length & weight, they have less inertia in the top part of the blade, diminishing the force at the end of the blade, but they still seem able to do it. It's a safe assumption that a Spartan hoplite would have considered that a sword.

Axes use a similar principle, but utilize the haft to generate the force. Cleavers might technically be swords, per the extreme weight to length ratio.

Not every cut in sword utilizes this action, but swords are the only bladed weapon with enough weight and leverage to utilize the action of the blade itself to multiply force.

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    Hmm... that would probably put kukris in the same sword category. Aug 17 at 15:08
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    @MacacoBranco Kukris has the weight and length. At 2 lbs, that's close to a medieval knight's sword. Kukris is short but blade heavy, a clear force multiplier when chopping. (Been in love with that knife-that's-probably-a-sword since the first time I saw it.)
    – DukeZhou
    Aug 18 at 0:54

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