The rachis is the rigid, upper central shaft of a bird’s feather that supports the flat, fan-like surfaces used for flight, insulation, and display. Made of lightweight and durable beta-keratin protein, it acts like a structural mast to hold the rest of the feather components together. Anatomy of a Feather
To understand the rachis, it helps to see how it fits into the broader structural anatomy described by Bird Academy: Rachis | feather part – Britannica