Originally Posted by
Anvilfire
Eer huh ? What is it?
It's called hi or bohi in Japanese, and its purpose is to allow for a lighter blade without weakening it substantially, as a blade that was thinner in its entire cross-section would be. European swords use similar grooves for the same purpose, and the English term for those grooves is Fuller.
Removing metal only from the shinogiji allows for a thicker cross-section for a given weight, and in addition to allowing for a stiffer blade, it also allows for better shaping of the edge profile (called niku).
Some people worry that a bohi may have been added only to cover flaws, but since the position of the hi is relatively fixed that is not as likely to be the case as with decorative carvings, which often do, unfortunately, exist to cover flaws (kizu).
Good swords can be found both with and without bohi, but Iaido practitioners in particular tend to favor bohi because the sound a blade with bohi makes can be used by the experienced to gauge their form; a helpful tool when one isn't getting feedback from cutting targets.
HTH.
Yours in Budo,
---Brian---