The Book of Changes. Sixty-four hexagrams, each with its moving lines, describing a situation and the direction it is already travelling. Cast for a question, or studied line by line as the source the other five systems draw their logic from.
A hexagram is built from the bottom up. The lower three lines describe the inner situation, the upper three the outer. Each line is yin or yang, and some lines are moving — in the act of turning into their opposite.
The cast hexagram is where you stand. Change the moving lines and you get a second hexagram: where the situation is heading if nothing intervenes. The reading lives in the distance between the two.
The Yi Jing is read as a text as often as it is cast. Its trigram logic underlies the eight directions in Feng Shui, the nine palaces in Qimen and the elemental relationships in Bazi.
Three coins, six times, in the traditional manner. Write your question first, in one sentence with a subject and a deadline, then cast once.
The caster gives you the hexagram numbers, names and moving lines. The judgement and line texts belong to the book — read them in a translation you trust, or bring the numbers to a session.