A kei jidōsha, light car, may be no more than 3.4 meters long, 1.48 meters wide and 2.0 meters high, with an engine of no more than 660 cc. The class is defined under the Road Transport Vehicle Act. The 660 cc limit dates from 1990. The present length and width took effect in October 1998.
Inside that box, Honda built the N-BOX around its cabin. In a Honda magazine interview, the chief developer of the first N-BOX describes the task: Shitsunai o hiroku suru tame ni, enjin rūmu o chiisaku shinagara, dō yatte kūkan o kakuho suru ka. “To make the cabin wide, how do we secure space while making the engine room smaller?”