There is a folk saying that "Dan Shu is ominous". Why do some people think it is unlucky to receive a letter written in red ink? It has its cultural and historical background. The word "Dan Shu" has three meanings: first, the so-called "heavenly book" fabricated by ancient rulers was written in Dan pen, so it was called "Dan Shu" and was taboo among the people. Second, the emperor often used "Dan Shu" to issue preferential treatment cards to heroes, and the exemption certificate was also written in red ink. These two meanings are far from us. However, it directly influenced people's ideas and caused the psychology of "Dan's ominous pen", because in ancient times, the names of criminals were recorded with Zhu brush. Up to now, criminals who have been sentenced to death have marked their names with an "X" or "√" in red ink. Therefore, people avoid writing letters with red pens. In addition to the above factors, writing a letter with a red pen is now considered to mean breaking up.