In case you don't know, I Ching is both a book, a metaphysical model of the universe, and a system of divination, originating in China. It has fascinated western esotericists from the time the first Jesuit missionaries heard about it in the 17th century (and became convinced the existence of the I Ching "proved" that China was an antediluvian culture, as the I Ching was so sublime that in their minds it had to be the product of the secret wisdom God revealed to Adam). It really took off in the 1960s, however; though as with many other things, Aleister Crowley got there first.
Here are some fun details:
-The I Ching is in fact the oldest known occult teaching in continual common usage in human history; for at least the past 5000 years. The Tarot, by contrast, is only about 400 years old.
-The I Ching book itself (more correctly pronounced "yijing", and sometimes called the "zhou yi"; which is actually the name for the core book without the Confucian Expansion Pack) is the world's oldest book still in common use. The earliest surviving copy we have dates to 200bce, but scholars generally agree that the book exists since 1100bce, though many think the first written copies could have been even older in more protypical forms.
-The system of the I Ching is based on a complex Taoist metaphysic, a conception the universe that begins with the idea of absolute unity (Tao) separating itself into duality (Yang, represented by a solid line; and Yin, represented by a line with a gap in it), then into combinations of those two in different proportions (expressed by different combinations of three lines, each either "yin" or "yang") to make up 8 elements (four celestial forces and four terrestrial forces). In turn, these eight elements mingle in pairs to form 64 possible "hexagrams" (so called because they're the combination of two 3-line figures).
-The system of the I Ching contains within it complex mathematical concepts: the I Ching system is the oldest known example of a binary code (4500 years before Liebnitz, who, by the way, was a student and admirer of the I Ching). Its 64 different hexagrams can be combined into different logical progressions that contain surprisingly sophisticated mathematical ideas: besides the binary, it was only recently discovered that one of the formats of the 64-hexagram sequence in fact expresses a projection of a "six-dimensional hypersphere". I'll be honest when I tell you that I don't myself entirely grasp that concept, I only know that some I Ching math-nerds are shitting themselves about how amazing that is. Another I Ching mathematician has noted that the sorting of the combinations forms something called "boolean lattice", which is also apparently pants-shittingly impressive.
-the progression of the I Ching, whatever else mathematicians might say about it, was explicitly stated to be meant to represent the development of TIME. So it is not just an esoteric "scale model of the universe" but a scale model of Space AND Time. "I Ching" literally means "The book of Changes", because the I Ching examines how weak forces and strong forces advance or decay over time in structured patterns.
-The original hexagams were said to have been discovered on the back of a turtle shell by the (mythological) Chinese emperor Fu Xi (who would have ruled sometime around 3100bce), who was also said to have invented writing, fishing and marriage. It was generally thought that it was another (this time non-mythological) monarch, King Wen of Zhou, who first provided the detailed explanation of the hexagrams that became the core text of the I Ching (thus, the "Zhou yi"). According to some version of the account, he is said to have devised the book while imprisoned (by a rival king).
-The rest of the I Ching, the "Commentaries" are attributed to Confucius, though some scholars question (as scholars endlessly do, to get paid) whether it wasn't just someone from the Confucian school who wrote them and not Confucius himself. In either case, there's no question that Confucius was a great admirer of the I Ching and set it up as to become one of the "Five Classics" of Chinese knowledge; Confucius was said to have once declared as an old man "If I could live just a few more years, I would dedicated 60 of them to the study of the I Ching".
-Aside from the Jesuit latin texts, the first translation of the I Ching to a european language was into English by James Legge in 1882. However, it should be noted this translation was very poor by modern standards, and crucially it provided no instructions on how to actually USE the I Ching as a divinatory text. That would have to wait until Richard Wilhelm's German edition, in 1923, but this would not be translated into English until 1950.
-None of that stopped Aleister Crowley, however, who has the distinction of being the first known white man to actually use the I Ching for divination! And he did so on a very regular basis from when he first discovered the I Ching while riding a donkey straight across China in 1905, until his death some 42 years later. His magical diaries indicate that he vastly preferred using and confiding in it over the tarot, which he found less specific; for long periods he would be consulting the I Ching almost daily, sometimes multiple times a day. Crowley wrote his own poetic version of the I Ching (drawn from Legge's version), but it was never printed in his own lifetime; he was also unable to find someone to teach him the right way to engage in I Ching divination, and ended up creating his own unorthodox system for performing the divination, probably based on an imitation of what he saw performed in his travels in China. On the other hand, he was one of the first westerners to approach the philosophical concepts of the I Ching as something valid and not poppycock, and he figured out the correspondences of the 8 trigrams to their elemental equivalents in western magick and thus their place on the qabalistic tree of life (as part of his, and many other magicians, endless quest to try to have a "grand unified field" of all magical symbolism).
-The I Ching became truly big in the west only in the 1960s, when a new edition of the Wilhelm translation came out in english (english translation done by Cary Baynes) with an introduction by Carl Jung. The Hippies discovered the book and it exploded in popularity, with potentially hundreds of thousands of teenagers using the I Ching really poorly. You also saw a kicking off of a number of new versions of the I Ching, some really excellent (like John Blofeld's), and others utter new-age crap that were radical reinventions of the book to fit new age philosophy. By the 80s you had a series of spin-off material like I Ching-themed Tarot Cards, other kinds of oracle decks, and of course, shitty chinese coins being sold for ridiculous prices at new age bookstores. As China opened up to the west, you also started to get some new and very serious translations that I think could now approach being called "definitive" (though often very academic and not very poetic), and the release of I Ching related books and texts from China that introduced new concepts to the west, like how I Ching relates to Chinese Astrology.
-Carl Jung loved the I Ching, utilizing it for a great deal of his own life; and he considered it one of the finest tools for the study of the self. Among other celebrity fans of the I Ching were William S Burroughs, Herman Hesse, Jack Kerouac, Alan Ginsburg, Ken Wilber, Paulo Coelho, Philip K. Dick, Jorge Luis Borges, John Cage, Bob Dylan, and no doubt many more.
-The famous new-age writer and psychedelic drug advocate Terrence McKenna claimed that the I Ching was a device to measure "novelty", in a series of mostly-incomprehensible explanations that had nothing to do with the traditional I Ching (shorthand: he was high). He claimed that as a measuring-system for "novelty" it indicated that all novelty would run out on December 21, 2012, leading to the eschaton, the end of normal time and space as we know it.
Unfortunately, McKenna didn't live long enough to see his bullshit proved wrong, as he died in 2000. However, thousands of the stoner-branch of new-agers continue to worship this appealing, romantic and charismatic utter moron, giving one explanation or another as to why "timewave zero" didn't end up happening, just hasn't happened yet, or sometimes 'has happened but no one noticed, man'!
-The most common method of using the I Ching is by tossing three coins, using the heads-or-tails positions of the toss to determine either a solid Yang line, a broken Yin line, or a Yang or Yin line that's "Changing" (that is, in the process of becoming its opposite). Doing this six times produces a hexagram, then flipping the changing lines to their opposite provide the "future" hexagram. The relevant parts of the I Ching which are then read are the main description of both hexagram plus the special description of those specific changing lines.
However, it should be noted that the "coin" method is not the real traditional method of using the I Ching and it does not generate the probabilities that are inherent in the original methods; with the coins the probabilities are weighted equally for yin and yang, but in the older conception there are varying probabilities for each of the four possible results, since change does not just happen with equal likelihood.
The traditional method is done using fifty thin sticks or yarrow stalks; but this is a long and complicated method most people today would not care to engage in. Of course, really serious purists will demand to do it, and to do it with the correct ritual fashion (just like at the other end of the spectrum, the total non-purists will go and buy themselves a pack of "I Ching Cards" and just use that instead because fuck complex mathematics and 5-millenium old secrets, I just want to play fortune telling!)
Some hardcore magicians, on the other hand, choose a more pragmatic approach, seeking to use a method that imitates the probabilities of the yarrow system without having to actually use the complex yarrow-counting process. As such, methods have been devised using 4 sticks (my favorite and preferred method) or 4 coins, two coins, or even 16 beads or tokens of different colors.
Characters in RPGs who use the I Ching could run the gamut from psychedelic stoner hippies, vapid wooly-brained new-agers, 'beautiful-mind'-style fucked up mathematics PhDs who've fallen down the rabbit hole, obsessive orientalists one bad night away from dressing up in a Mandarin outfit, or of course hardcore ceremonial magicians. Go nuts.
Currently Smoking: Lorenzetti Solitario Rhodesian + C&D's Pirate Kake