Air-conditioning dates back to prehistory. Ancient Egyptian buildings used a wide variety of passive air-conditioning techniques.[4] These became widespread from the Iberian Peninsula through North Africa, the Middle East, and Northern India.[5] Similar techniques were developed in hot climates elsewhere.[further explanation needed] Passive techniques remained widespread until the 20th century, when they fell out of fashion, replaced by powered A/C. Using information from engineering studies of traditional buildings, passive techniques are being revived and modified for 21st-century architectural designs.[6][5] An array of air conditioning condenser units outside a commercial office building Air conditioners allow the building indoor environment to remain relatively constant largely independent of changes in external weather conditions and internal heat loads. They also allow deep plan buildings to be created and have allowed people to live comfortably in hotter parts of the world.