木直中绳[ mù zhí zhòng shéng ]

⒈  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。

例木直中绳，輮以为轮，其曲中规。——荀况《劝学》

英a timber (lumber) is as straight as a straightened line made by a carpenter's ink marker;