I Ching, Yijing or Zhou Yi
"Oracle of the moon": © 2000 LiSe
What does it mean, 'inner' and 'outer trigram' (popup)
Thunder below Fire: Thunder has the power to make things happen or change. It knows no doubts or misgivings. Fire shows what is involved, gives insight in facts, separates truth from false. Together they can make things happen in a clear and focused way, showing the borders and limits and the right spot to hit.
Ideogram of the hexagram name: the first (upper) character, Shi, is mouth, bamboo and a shaman's tool (筮 divination by milfoil). Maybe the one who "bites in and picks things apart". The lower, Hé, is a mouth and a vessel with content and a cover: bite through, closing the upper teeth over the lower teeth. The Mawangdui Yi has 筮 instead of 噬. It is possible that this is the old version in the Yi as well, dating from ancient times, when divining took preference over discussion or human advice.
The 'Great Image' says: Thunder and lightning: Biting through. The ancient kings made the penalties clear to enforce the law.
Hex.21 is the contrast of 48, which is about working together in a social way.
Hex.21 is about laws and rules which have to be followed, so a community will be safe and ordered. No one is allowed to harm another, and this is enforced by punishing misdeeds in a clear way.