The function of both types of reducer is to reduce the line from a larger to a smaller pipe size, this obviously results in an increased flow pressure. With the eccentric reducer the (smaller) outlet is off center to the (larger) inlet enabling it to line up with one side of the inlet and not with the other making it deal for horizontal piping applications. The concentric reducer is manufactured that both inlet and outlet are in a common center point which can be suitably used for vertical piping needs. The concentric reducer is easier and less expensive to produce but does not allow quite the same versatility as the eccentric reducer. The lengths of both types are fixed by manufacturing standards.