Sori measures the curvature of a Japanese sword.
Essentially, to measure the curvature of a Katana, imagine a straight line between the munemachi and the kissaki, then - from the deepest point of the curvature, trace a second imaginary line until it intersects with the first one.
There are bascially three types of sori - the one pictured above is known as Toori sori and is the most common, with the curvature deepest in the middle of the blade. A sword with the deepest point of the curvature towards the tip is 'sakisori' while one with the curvature towards the handle is called 'koshisori'.
A good curvature is typically somewhere between 1/2 to 1" - with a maximum curvature of approx 1.25" to 1.5" at the absolute extreme.
It is a common misconception that the more curvature a blade has, the better it cuts, however this simply is not true - for a sword with too much curvature will actually cause the tip to twist when it contacts a target - resulting in more bad cuts that usual.
Essentially, how much curvature a sword should have (within what actually works) is a matter of personal preference.
Watch the video below for an explanation of why a Katana is curved and what effects it has on cutting.
The curvature of a Japanese sword is actually made during differential hardening - the softer spine is pulled backwards as it is quenched, with how long it is held in position when quenching determining the extent of the curve.
Swords that are not clayed are simply shaped during the forging process in the same way any other curved sword (such as a cutlass, saber, etc) is made.
I hope this information on Sori has been helpful. To return to Samurai Sword Terminology from Sori, click here