This fitting connects one electrical product to another through a knockout ranging from 3/8" to 6". It functions like a nipple except it is formed with a flat locknut affixed at one end. You'd use it to connect fixtures together to form continuous runs, connect metal boxes together, etc. The center of the Chase nipple is hollow and they are available with an insulated throat. Named after this turn of the century manufacturer. Sometimes gets confused with Close Nipple which is completely different.