The door threshold forms the lower closure of an exterior door and serves several simultaneous functions: it seals against rain, wind, cold, and sound, drains water away, and protects the door rebate from mechanical damage. In exterior applications, thresholds must be designed to manage water drainage or be pressure-watertight. Barrier-free design according to DIN 18040 requires threshold heights of no more than 20 mm or fully level transitions. Modern low-threshold systems with integrated brush seals or drop-down sealing strips simultaneously achieve high driving rain and wind resistance ratings. Materials include aluminium, stainless steel, or plastic. Correct installation alignment and drainage are critical for the long-term functionality of the entire door assembly.