The I Ching, or, in English, Book of Changes is an ancient Chinese system of divination. An oracle is cast by flipping coins or more traditionally, by manipulating yarrow stalks. The oracle is one of 64 different hexagrams. The hexagram itself is composed of two trigrams, each consisting of three lines. Those lines are either straight (or Yang) or broken (Yin).
The Yang lines look like:
The Yin lines look like: