Tatami Forensics

Tatami forensics[44] refers to the part of cutting calibration in which you evaluate your results by studying the tatami target. Cutting calibration is very similar to FBI ballistics testing. When people test guns on ballistic gelatin, they often believe, mistakenly, that the gelatin is representative of the human body and that what a bullet does in gelatin is what it will do in the body. For example, they believe that if a bullet penetrates twelve inches into gelatin, it will penetrate twelve inches into a person. This is a very popular misconception, and it is very similar to the misconception people have about cutting tatami.

The FBI standard (twelve to eighteen inches of penetration in a very specific kind of ballistic gelatin after penetrating four layers of denim) is based on comparing results in gelatin to results in real world shootings. A bullet that penetrates fifteen inches into the gelatin will not penetrate fifteen inches through clothes, flesh and bone. But the FBI knows that a bullet that can penetrate fifteen inches into their gelatin will perform a certain way in the human body based on the correlation they have established between the gelatin penetration and real world performance.

Here is a quick and dirty (oversimplified) example. Let’s say you want to test the effectiveness of several different calibers of defensive ammunition. First, you need to establish a correlation between your test medium and real world performance. So you take a round that you know for a fact is very effective, say a .357 Magnum, and shoot it into a block of ballistic gelatin that you cooked up following a very specific and repeatable formula. Let’s say that the .357 penetrates twenty inches into the gelatin. You then take a round that you know from field reports and medical data is the bottom edge of effectiveness, say a .32 auto, and shoot it into the same gelatin. The .32 penetrates ten inches. You have now established that to be effective, a bullet must penetrate ten or more inches into your specific formula of gelatin, and that a penetration of 20 inches is an excellent result. You now have your correlation and can begin your testing.

You test three calibers, .25, .380 and .45. The .25 penetrates eight inches, the .380 penetrates twelve inches and the .45 penetrates eighteen inches. You can deduce, based on your established correlation, that the .25 is not effective, that the .380 is marginally effective and that the .45 is very effective. You can also add wound channel diameter to your calibration if you have enough data, but that is unnecessary for this example. If you change the formula with which you make gelatin, your results won’t mean anything until you establish a new correlation. Everything has to be as consistent as possible for your data to have value.

Similarly, tatami can inform us about what our cut would do in the real world if we know how results on tatami correlate to real world results. This correlation has been established by the Japanese, and this is a huge part of what makes tatami such a valuable cutting medium.

Tatami reveals mistakes in two important ways. The first is the appearance and shape of the severed pieces, primarily the one still left on the stand. The second is the behavior of the piece that is severed from the main body of the mat (that one still attached to the stand).

Not all of the criteria for a successfully executed cut apply to all cuts. Most of the diagnostic information in this section applies to basic ascending, descending and horizontal cuts, done with the long edge. These are the cuts you should start with and learn to read before moving onto more advanced cuts. Also, not all of these standards apply to swords held in one hand. Such exceptions will be noted.

Successful basic long edge ascending, descending and horizontal cuts, when performed on a tatami mat, should look like the following images when looked at from directly in front or directly behind the target. The first depicts a successful diagonal descending or ascending cut, the second depicts a successful horizontal cut.

The cut should be straight and flat (not slanted towards or away from the person cutting). The severed surface should be smooth. The severed piece should fall gently away and not fly violently in any direction.

Once these goals have been set, you can start to diagnose your failures and correct them. The following is a list of common mistakes, along with an explanation of what they mean and suggestions on how to fix them. This list is by no means exhaustive, but it is sufficient to build a solid understanding of diagnostic calibration.


The simplest and most common issue is aim. Aside from the obvious visible manifestations of poor aim (missing the mat, striking the stand, hitting too high or too low, etc.), one that is not quite as obvious is the “mountain.” If you look at the image below, you will see that the cut hit the area of the previous cut and made a little peak in the mat, like a roof or a mountain (hence the name). When you hit the area of your previous cut, you are cutting through less mat than you would otherwise. You are also leaving less surface area from which you can diagnose your cut. Keep in mind that a mountain is only a mistake if it is unintentional.

There is no specific way to fix this mistake except to improve your aim through practice. Working with a pell is a great way to improve your aim.


This is another very common mistake. A scooped or scalloped cut is one in which the trajectory is not perfectly straight, which results in either a failed cut or, if successful, a cut that looks like the following image:

People scallop cuts for a variety of reasons, some physical (resulting from a flaw in technique) and some mental (wherein the fencer unconsciously alters the trajectory). One reason a very inexperienced fencer might do this is to avoid an unpleasant sensation. Imagine striking someone diagonally, with a steep angle, on the side of their fencing mask. It is a feeling that, for some people, is akin to the sound chalk makes when scraped along the black board. The sword skids down the side of the mask and the resulting feeling of impact is profoundly unsatisfying. So when hitting someone in the fencing mask, most people prefer to do it either vertically (straight down) or horizontally, which results in a more solid and satisfying impact—and so they transfer this way of striking to the tatami mat. This comes from being habituated to striking objects with blunt swords, sticks and/or clubs, and for some it can be a hard habit to break. In others, this comes from seeing a vertical tube-shaped target and deciding, consciously or otherwise, that the best way to cut it is to transect it horizontally. Both result in a trajectory that starts out straight in a downward diagonal path and then curves as the fencer unknowingly attempts to transform it into a horizontal path. People who are unfamiliar with how swords work have a hard time understanding that sharp swords do not work like blunt swords. They will not skid down the target, unless your edge alignment is off or your grip is poor. The only way to defeat these impulses is through practice and forming muscle memory, as well as cutting targets and developing an intuitive understanding of how swords work.

One technique that you can use to overcome this in the short term is to strike the tatami mat lightly along the same trajectory that you are trying to use to cut it. For example, slowly swing the sword in a straight 45 degree path and lightly bite into the tatami mat several times. This “sets” the trajectory in your mind and then when you cut, the trajectory is usually straight. This goes away on subsequent cuts unless the process is repeated. This is most common in students who do not do much solo training.

Using the shoulders and/or not keeping center is another common cause of scalloping. As mentioned in the Theory module, using the shoulders causes one shoulder to cock up and the other to drop, which causes your body to pivot, and makes you lose center. Likewise, not keeping center causes you to move your shoulders and pivot in the same way—it’s the same thing, the only question is what causes it.

Another reason for scalloped/scooped cuts is that grip power is applied while the sword is passing through the target and not just before contact as it should be, though that more often results in “halfway cuts” (see the Tatami Forensics section). Conversely, a weak grip in one hand or a bent elbow in one arm can also cause this, and it can be exacerbated if your edge alignment is slightly off.

To figure out which of these issues is causing scalloping in the absence of a qualified instructor, you will need to watch yourself cutting. Make a video, preferably slow motion at 240 frames per second or more, and examine it closely. Are your shoulders rocking and causing your body to pivot? If so, where does that motion originate? If you are fixated on the target, you will turn your sword with the hands first, and then the shoulder will follow. If your shoulders are rocking first, and then your sword follows, then you will need to stop calibration and return to practice, working on not using your shoulders. One of the most common reasons people cock their shoulders is because they cannot make angled cuts without leaving center. Go to the Centering section and make sure you are creating your angles correctly. If your problem is something other than this and you can’t figure out how to fix it, seek help from an instructor. If you don’t have anyone local, go to a seminar or workshop. If none of these are an option, then read this book again and practice all of the things it describes to the best of your ability. If you do everything right, you won’t need to figure out what you’re doing wrong.

Slanted Cuts

This applies only to basic long edge ascending and descending cuts as well as basic long edge horizontal cuts, and primarily to swords held in two hands (longswords). Specialty cuts (e.g. Zwerchhau) need not conform to this standard. When you look at the result of a basic cut, the surface of the severed mat should be parallel to the ground along the Y axis, as shown in the image below.

A perfect cut, when viewed along the Y axis, should look flat, like the image below on the left. A cut is slanted when it looks like the image on the right. Note that the slant can be in either direction. Regardless of which side of the target you are looking at, the mat can slant/slope either away from you or towards you.

The most common reason cuts sometimes end up slanted in this manner is that the person cutting is either pushing or pulling sideways with one hand. This can be the result of uneven grip pressure or keeping one arm bent while the other is straightened (usually the left elbow in a right-handed fencer will be bent when this happens). This compromises that person’s structure by creating uneven pressure with the hands. To overcome this, focus on even grip pressure and keeping both arms fully extended.

Although slanted cuts may not seem bad as they can be otherwise perfect, they are indicative of major problems that will manifest in more advanced cuts.

Running Out of Sword

Sometimes you will deliver what appears to be a perfect cut, and yet the mat will remain standing even though your sword passed through it cleanly. Upon examination, you will see what looks like a good cut that only goes part way through the mat. If there are no other signs of failure, then this is called “running out of sword.” The cutting arc, as a consequence of the geometry of human biology, pulls the sword along the target as it progresses, drawing it closer and closer to the wielder until it exits the target. If you cut too close to the tip, there is not enough blade length being brought into play to sever the mat, even if the point of the sword is past the target when it makes contact. Get closer to the target and try again.

Halfway Cuts

Halfway cuts start out fine but something happens in the middle of the cut and it goes bad. You will see that a portion or even the majority of the cut is perfectly straight and smooth but then the rest of it is rough, scalloped or frayed. If the cut failed (the sword did not go all the way through the mat), then the most common cause of this is a lack of velocity, power or structure. The edge alignment was good, the trajectory was good, but the person cutting didn’t have enough follow-through to finish the cut. To correct this, focus on generating power from your body, using the hips to power your strike rather than the arms or shoulders. This usually doesn’t happen to strong people because they can muscle through a tatami mat, and when they fail, the failure manifests in different ways (usually scooping). This could also result from a loss of control (see below).

If the cut was successful (the sword severed the target, as shown in the image below), the most likely culprit was a loss of control (grip) or the application of grip power in the middle of the cut.

Both of these are grip issues that should be corrected by practicing with an emphasis on proper grip and correct application of grip power. Remember the warning about grip power, however. Only attempt to learn how to time it closer to the point of impact once your cutting skills are well developed. When you’re just starting out, don’t try to do it. If you’re having this particular issue, then don’t try to launch the sword with grip power, instead keep an even and steady grip on the sword throughout the arc of the cut. As your cutting becomes consistently better, start to apply grip power at the start of the cut.

Another common failure of this type is when the cut starts out good but then turns in the target and starts descending through the mat, along the grain, in a straight line, and eventually comes to a stop, as shown in the image on the right. The most common cause of this failure is a weak grip, which manifests as lack of structure. The rest of your structure is irrelevant as your grip is what connects you to the sword, and structure fails at the first weak point. Just as a car with flat tires will drive and handle very poorly, regardless of how good the rest of it is. If this happens to you and you cannot fix it by tightening your grip, you will need to work on grip strength with weights, weighted trainers and other tools. This can also happen if your grip is fine but you lack follow-through and cut to the target rather than through it, though this will usually manifest as a completely failed cut.

Rough Cuts

A rough cut is just that, rough. The sword severs the mat, but the target surface is irregular. Usually tatami debris goes flying in all directions, the stand wobbles and the severed piece catapults into the wall as though it were launched from a trebuchet. This is one of the things that can happen when a strong person muscles his or her way through a cut. The edge alignment is off, the trajectory is probably not straight and the person is making all kinds of other mistakes. But he or she has enough power to force the sword through the target regardless, because tatami is not all that resilient and enough power can overcome minor problems (clothed human bodies are not as forgiving).

Blade flex has a lot to do with it as well. A jarring impact with imperfect edge alignment can result in vibrations and blade flex that further hamper the progress of the cut. A sword that is too thin being violently torqued with grip strength may also present this issue even though the fencer’s technique is otherwise perfect. Know your sword, and know how far you can push it before this happens.

There is no quick fix for rough cuts. Start by reducing your power to make it easier to identify your primary mistakes and work to resolve them through cutting practice before trying to calibrate again. When you reduce power, you will most likely find that you will fail to sever the mat. Start by working on edge alignment and progress from there.

Flying Tatami

One of the most important bits of diagnostic information about your cutting can be obtained by watching what happens to the severed piece of tatami as you cut it. This is another criteria that only applies to basic cuts, and only ascending and descending cuts. If you want to apply it to more advanced cuts, you have to factor in a lot of things that will become obvious to you as your skill and experience grows. Attempting to describe such things here would be counterproductive.

Ideally, in all cuts, the severed piece should remain in place, still standing. This is an almost impossible standard, though it does happen on rare occasions. A more realistic standard would be that the severed piece stays in place for a split second then gently falls away, or falls right away but does so smoothly and gently.

If you are cutting with a descending diagonal cut from the right, the piece should fall gently away from your cut, from right to left. The opposite is true for cuts from the left.

If the severed piece falls forward, it is usually because your tip was trailing. See The Arc in the Practice module to fix this. It could also mean that the focus of the arc was too low (see the Understanding the Cutting Arc section).

If the severed piece goes flying to the left when cutting from the right (or to the right when cutting from the left), that means too much force was exerted laterally during the cut. If this is not tied to another problem such as scooping/scalloping or halfway cuts, then the issue is that the sword is being pushed to the side during the cut—aside from movement along the trajectory, the sword is also moving laterally. Sometimes this happens because of excessive hip rotation that is not coordinated with the upper body. Focus on your trajectory during practice and work at eliminating this excessive pushing.

Tatami Forensics Conclusion

It may seem strange that the tatami forensics section isn’t a lot bigger than it is. After all, all of the information in this book was geared towards improving body mechanics which mostly manifest in the cut, and tatami forensics is what allows you to diagnose your problems and fix them. In practice, however, the specific issue you are having should not lead you to discover some massive flaw in your technique. Correct technique is easy to see without cutting calibration. Are you extending? Are you centered? Do you stay centered? Are you in balance? Are you grounded? And so on. If, after reading this book and understanding what good form looks like, you can spot issues in your technique, then you should correct them before you begin cutting calibration.

If you dip your shoulder and pivot your upper body, you can see that clearly in a mirror or on a video recording. You don’t need to waste time and money cutting tatami to know that you will scoop your cut. If you can’t get good sword wind in your cuts, then you don’t need to cut a mat to know that you’ll probably screw it up. Fix your major issues before you begin calibration, and use tatami forensics to find the sort of things you can’t see in a mirror when cutting air.

Edelson, Michael. Cutting with the Medieval Sword: Theory and Application (pp. 148-154). Kindle Edition.


[44] This section has been edited and added to by Sang Woo Kim, who probably knows more about cutting and tatami forensics than any other person in the Western world. He also coined the term “tatami forensics.”

Edelson, Michael. Cutting with the Medieval Sword: Theory and Application (p. 163). Kindle Edition.

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