Takouba with five fullers

[Helleri]

I'm going to go ahead and suggest that 5 fullers was a mechanical choice. For any not aware, the purpose of a fuller is to selectively strengthen a blade against flexing against it's width along it's long axis without adding or subtracting weight (ideally they are a hammer work feature).

While popular myth holds that they are "blood grooves"; That somehow pocket air or allow blood to flow making a blade easier to cut or thrust with. A wet handle from blood or more surface area causing less friction while passing through an elastic medium, simply makes no sense, and to the best of my knowledge has never been demonstrated to be the case.

Some things we do know about fullers from stress tests and photoelastic photography (looks at optical changes in a material through deformation) is that they seem to work best when dome/half cylinder/semi circle shaped in cross section; When spacing from another is not greater than the width of one; When depth is equal to the width of one.
[Note: if yall really want me to, I can dig all that info up again, but it's a few computers back now and I am in process of moving so might be a while.]

We also have to consider that different blades vary in width and ultimate thickness. So it's easier to put larger and fewer fullers on a thicker blade that has a strong lenticular, wedge, or diamond shape. Because thicker means they can afford to be deeper, thus wider, and thus fewer.

personally I think this blade has the right amount of fullers for it's apparent width vs thickness and what it looks like they were trying to do, which was probably to have differential flexibility throughout the blade but with uniform tempering and heat treatment. Or in other words the maker seems to have been looking to do the most mechanically with the least work. Hot forged fullering is pretty quick and easy compared to careful differential tempering and heat treating.