A fitting is an element composed of a nut and a ferrule that are specifically designed to fit together. Fittings are most of the time screwed to receiving ports. When it’s locked in, the ferrule seals the tubing to the nut which is itself hermetically sealed to the port. Two fittings might have the same thread, but different port geometries, OD tubing diameters or Head geometry. Though Fittings are intended for high-pressure connections, they are usually also used in low pressure microfluidic applications.