What is coin with square hole in middle?

A coin with a square hole in the middle is known as a Chinese cash coin or a Chinese charm. They were used as currency in China from the 4th century BC until the 20th century. The square hole in the center allowed the coins to be strung together and carried in large quantities. They were made of copper and varied in size and design depending on the era and region they were produced in. The inscriptions on the coins often contained the name of the ruling dynasty and the reign period. Today, Chinese cash coins are often collected for their historical and cultural significance.