木直中绳　

◎ 木直中绳 mùzhí-zhòngshéng

[a timber (lumber) is as straight as a straightened line made by a carpenter‘s ink marker] 木材直得合乎拉直的墨线。中,动词,符合、合乎。绳,木匠用以取直,浸满墨汁的线绳

木直中绳,輮以为轮,其曲中规。——荀况《劝学》