[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線。中,動詞,符合、合乎。繩,木匠用以取直,浸滿墨汁的線繩

木直中繩,輮以為輪,其曲中規(guī)。——荀況《勸學(xué)》