The Samyutta Nikaya, the Collection of Connected Discourses, organizes thousands of comparatively short suttas not by length, as the Digha and Majjhima do, but by shared theme or shared interlocutor, gathered into 56 thematic groupings, or samyuttas, covering subjects from the workings of dependent origination to specific conversations with particular deities, monks or wanderers, giving the collection an encyclopedic, cross-referencing quality distinct from the other nikayas' more sequential presentation. Its single most consequential text by any measure is the Dhammacakkappavattana Sutta, "Setting in Motion the Wheel of the Dhamma," traditionally identified as the Buddha's first teaching after his enlightenment, delivered to five ascetics at Sarnath, laying out the Four Noble Truths, that suffering exists, that it has a cause, that it can cease, and that a path leads to that cessation, and the Middle Way between self-indulgence and self-mortification that the Buddha had tested and rejected both extremes of during his own years of ascetic practice.
Because this sermon marks, within the tradition's own account, the actual founding moment of Buddhism as a distinct teaching rather than merely one more ascetic philosophy among many contemporary rivals, its place within the Samyutta Nikaya gives this nikaya a claim to doctrinal centrality that its comparatively modest length and topical, cross-referenced structure might not otherwise suggest.