In some example pliable air duct systems, inflatable ducts of various diameters and lengths are created by selectively assembling pre-existing stock pieces in different combinations. In some examples, the stock pieces include disconnectable longitudinal joints and disconnectable circumferential joints, wherein the longitudinal joints enable interconnecting multiple stock pieces to achieve a desired tube diameter, and the circumferential joints allow connecting multiple tube segments end-to-end to produce an air duct assembly of a desired length. To control the volume and/or the direction of air discharged from the duct, the duct assembly, in some examples, includes an adjustable register comprising a movable pliable sheet that overlies a discharge opening in a pliable sidewall of the duct. In some examples, the inflatable duct includes one or more cutout patterns on the duct's sidewall to provide guidance in creating a sidewall discharge opening of a proper size and location.