The Patthana, the Book of Conditional Relations, is by most reckonings the longest single text in the entire Pali Canon, running in some standard printed editions to many thousands of pages, and it stands as the Abhidhamma Pitaka's culminating achievement, taking the phenomena the Dhammasangani had painstakingly enumerated and systematically working through twenty four distinct modes of conditionality, paccaya, by which any given phenomenon can causally relate to and support the arising of another, root condition, object condition, proximity condition, and twenty one others, each precisely defined and then applied across the full range of possible combinations of mental and physical phenomena the earlier Abhidhamma books had classified.
Where the Dhammasangani offers something like a static inventory of every possible type of experience, the Patthana supplies the dynamic, causal grammar explaining how those experiences actually arise, condition and give way to one another moment by moment, providing what the Theravada tradition regards as the fullest possible account of dependent origination worked out at the most granular level of analysis the Abhidhamma method is capable of. Its sheer combinatorial scope, applying twenty four conditions across the entire matika of categories established in the first book, accounts for its extraordinary length relative to every other text in the canon.