The sorcerer, Clow Reed, created the Clow Cards in Hong Kong. They are made up of a combination of both Eastern and Western magic where each Clow card has their own unique role and special power. Some cards are friendly and playful while others tend to be mischievous. There are only four elemental cards and they tend to be the most dangerous. The four cards are Windy, Firey, Watery and Earthy.

The Clow cards, without a master, can do anything on their own as they have nobody to control them and this would only lead to chaos and trouble for humans. Although some of the cards have some intelligence and occasionally have their own personality e.g. The Mirror card. Non-magical people do not have the ability to see the Clow cards, which is stated in the manga.

Clow Reed sealed the Clow cards in the Clow book and he assigned two magical beings to guard the Clow book. Cerberus was elected the beast of the Seal and he was the only one allowed to elect who will be the next Master of the Clow cards and Yue, the Judge, has the task of judging the candidate to see if they are worthy of becoming the next master. Clow placed Cerberus at the front of the Clow book in the form of a Golden Lion and Yue on the back of the book as the Moon.