I Ching books come in a variety of formats. Its core text—the ancient Zhouyi (Chou-i)—consists of the sixty-four hexagram graphics and their short texts (the Judgment and the line readings).
Over two thousand years ago, a number of essays and explanations called the Ten Wings were added to the core text. The book as a whole was then renamed Yijing (Change Classic) and became part of the official canon.
The Wings have different characteristics and purposes: philosophy, mnemonics, hexagram interpretations. Two different traditions evolved for the book’s format. One was to have each part be a free-standing texts (such as in Rutt or Legge), another was to section up hexagram-related Wings and interperse those with the older layer of the Chou-i (such as Richard Wilhelm does). Note that some I Ching editions do not include the Wings, or may include only some. For the most in-depth study of the Ten Wings’ contents, history, and style, see Richard Rutt’s Zhouyi.
The structure of the Wings is as follows, using Richard Wilhelm’s I Ching format. All of the hexagram-related Wings are found in his Book III; the two philosophical Wings are in Book II.
Wing 1 & 2–Commentary on the Decision (彖傳 Tuanzhuan/T’uan Chuan). This Wing explains the core hexagram text known as the Decision or Judgment. In Book III.
Wing 3 & 4–Images. The Great Images is a commentary on each hexagram. The Small Images are commentaries on each line of a hexagram. The Great Image (大象 Daxiang/Ta Hsiang is in both Books I & III; the Small Image 小象 Xiaoxiang/Hsiao Hsiang is in Book III).
Wing 5 & 6–The Great Treatise (大傳 Dazhuan/Ta Chuan), also known as the Commentary on the Appended Statements (繫辭傳 Xicizhuan/Hsi-tzu Chuan). A highly influential essay on the meaning of the Yijing and the Universe. In Book II.
Wing 7–Commentary on the Words of the Text (文言 Wen Yan). Remnants of early commentaries on Hexagrams 1 & 2. In Book III.
Wing 8–Explanation of the Trigrams. (說卦 Shuo Kua/Shuogua . Discusses the symbolism of the trigrams (three-line components of hexagrams). In Book II.
Wing 9–“Sequence of the Hexagrams” (序卦 Xugua/Hsü Kua). A poem summarizing the meaning of each hexagram and why it connects to the subsequent hexagram. In Book III.
Wing 10– “Miscellaneous Hexagrams” (雜卦 Zagua/Ts’a Kua). A mnemonic for remembering hexagrams, however the order of hexagrams in this Wing is different (though still in traditional pairs, except for last eight hexagrams). In Book III.