This document discusses duct systems used for heating and cooling large commercial buildings. It outlines general design rules for ductwork, such as minimizing sudden direction changes and maintaining an aspect ratio close to 1. The objectives are to characterize energy losses in commercial building duct systems and develop methods to measure those losses. Properly designed duct systems provide benefits like adequate airflow, less equipment size/costs, better indoor air quality, and higher energy efficiency. Common duct materials include galvanized metal, aluminum, fiberglass, and flexible duct. Instruments like velometers and pitot tubes are used to measure air velocity and pressure. Insulation is important to limit temperature differences between the inside and outside of ducts.