A 90 deg register boot or PH2 is made to easily transition airflow from a round flexible or hard duct to a reactangular register or vent. The Longer kneck allows you to install easily in the ceiling or wall and easily connect your round duct. The airtight design also improves your heating, ventilation and air conditioning (HVAC) system’s overall performance quality. A reinforced mud ring allows for a durable fit. For smaller jobs, there are the 3 1/4 x 8-5″ through 14-8″ boots. Larger jobs may call for 4 to 6 x 10-4″ through 14-8″ 90 deg boots.