Classification of fittings and couplers used for piping connections in pneumatic systems. Includes one-touch fittings, threaded fittings, flange fittings, quick couplers, tube fittings, etc. that reliably connect air hoses to pneumatic equipment such as cylinders, valves, and filters. Essential components for pneumatic piping construction in manufacturing lines, equipment replacement during maintenance, and attachment/detachment of portable pneumatic tools, designed with emphasis on pressure resistance, airtightness, and workability. Distinguished from hydraulic fittings by working fluid and pressure range, and clearly differentiated from electrical connectors and mechanical coupling parts by connection method.