《梦溪笔谈·故事二》
古代的兼职官员，多数是暂时兼任，如有长期兼任者，就相当于正官。
古之兼官，多是暂时摄领；有长兼者，即同正官。
In ancient times, most of the concurrent officials were temporarily concurrent, if there were concurrent officials for a long time, they would be equivalent to regular officials.