The transition duct connects the duct from the dryer to the main ductwork that begins in the wall/ceiling/floor. There are a few things to bear in mind regarding transition ducts. Firstly, they should be of the appropriate material. This means that it should be a heavy metal material. Slinky foil ducts should not be used--ever--in the dryer vent system. Typically, the choice material for transition ducts is semi-rigid duct. This allows for heavy metal material while maintaining flexibility while connecting the dryer. Secondly, the transition duct should be of the appropriate length. Code requires that the transition duct not exceed eight feet. Unfortunately, eight foot sections of semi-rigid duct do not actually equal eight feet--they are often closer to six feet when extended. This means that in certain longer-length situations the main duct needs to be extended in order to bridge the difference. Thirdly, the transition duct should not extend out of the laundry room area. Non-rigid material certainly cannot penetrate past the wall/ceiling/floor of the laundry room such that it is concealed. Lastly, the transition duct should be listed as in accordance with UL2158A.