An old text with wings
The core of the I Ching, the Zhouyi, is short and cryptic: 64 figures, a judgment for each, and a text for every line. Over the centuries this core gathered commentaries that try to explain it. Ten of them became part of the book itself and are known as the Ten Wings, 十翼. They're called wings because they help the text take flight, opening it up to moral, philosophical and cosmic readings.
Tradition credits the Ten Wings to Confucius. Scholars today see it differently: they were written by several hands, probably between the Warring States period and the early Han dynasty, centuries after the core. The exact dates are still debated.
The ten, one by one
There are seven texts, but three of them come in two parts, which brings the count to ten.
1 and 2. Commentary on the Judgment (彖傳, in two parts). The Tuan explains each hexagram's judgment. It looks at the two trigrams and the lines in charge, and says why the hexagram means what it means. It's one of the first "technical" readings of the book.
3 and 4. Commentary on the Images (象傳, in two parts). This holds two kinds of text. The Great Image describes each hexagram's two trigrams as a scene from nature, then says what the noble person does in response: "thunder and wind… so the noble person…" The Small Images comment on each line in a short phrase.
5 and 6. The Great Commentary (繫辭傳, in two parts). This is the book's philosophical essay. Here you'll find yin and yang as the Way, the Taiji that everything springs from, the legend of Fu Xi drawing the trigrams, and a description of the yarrow stalk method. If you've ever read that the I Ching is a book of wisdom and not just of fortune-telling, that idea comes largely from here.
7. Commentary on the Words (文言傳). The Wenyan covers only the first two hexagrams, Activity and Passivity, in great detail, line by line, as a model for reading all the rest.
8. Discussion of the Trigrams (說卦傳). This one is about the eight trigrams: their qualities, family roles, animals, parts of the body and the two circular arrangements. Much of what we know about the eight trigrams comes from it.
9. Sequence of the Hexagrams (序卦傳). It walks through the King Wen sequence and explains why each hexagram leads to the next, as if they were chapters in a story.
10. Mixed Hexagrams (雜卦傳). It groups the hexagrams in pairs and defines them by very brief contrasts, almost like a memory game: two or three characters for each.
How they appear in a book
In most editions the Ten Wings aren't printed together at the end. The Commentary on the Judgment and the Commentary on the Images are split up and placed under each hexagram, next to the text they explain, and the Commentary on the Words sits with 1 and 2. The rest, the Great Commentary, the Discussion of the Trigrams, the Sequence and the Mixed Hexagrams, usually come as appendices. So when you open a hexagram, you're reading texts from very different eras side by side.
Archaeology shows that more commentaries were in circulation. The Mawangdui manuscripts, buried in 168 BCE, include a version of the Great Commentary along with other texts that never made the list of ten.
What they add to your reading
The Ten Wings changed how people read the I Ching. Because of them, a consultation isn't only "will this go well or badly?" but also "what attitude does this moment ask of me?" The Great Image in particular always ends in conduct: what a wise person would do in that scene.
In Cambios, the poetic layer of each hexagram is written from the classical text, which includes the judgment, the lines and the wings that comment on them directly. To see how these layers built up over the centuries, read The History of the I Ching.