In Japanese, the word 'koshi' is a mathematical term for equidistant segments and dividers. Generally, though, koshi is used to represent lattice doors or iron grates.

From olden times, Japanese lattice doors were doors of temple-style architecture. This changed during the late Heian period when double sliding doors became more popular. Black laquered sliding lattice doors are described in the 'Tale of Genji Picture Scroll' and the 'Annual Event Picture Scroll'.

Lattice doors can separate spaces, ventilate rooms, take in light and make rooms look more beautiful, all at the same time. All of these things connect to the introduction of shoji: paper sliding doors.