A rare opportunity to compare the inside and outside geometry of a finished handle core. Historically an old tsuka would be split open for repair or adjustment or even re-purposing for a new blade, however a newly crafted tsuka is always glued together before the outside is shaped.
In this unusual case the core was opened and adjusted at a late stage in shaping, providing an example to study and observe. The proportions in the example below are generally based on study of a range of antique sword tsuka, with the main difference being the kataki (hardwood) style lacking samegawa and silk cord wrapping.
More about the interior of a traditionally crafted tsuka and saya: