In this paper, we aim at obtaining insight into how a multivariable feedback controller works, with special attention to serial processes. Serial processes are important in the process industry, and the structure of this process makes it simple to classify the different elements of the multivariable controller. In particular, we focus on the difference between the feedforward and feedback parts of the controller. Feedforward control may improve the performance significantly, but is sensitive to uncertainty, especially at low frequencies. Feedback control is very effective at lower frequencies where high feedback gains are allowed. An example of neutralization of an acid in a series of three tanks is used to illustrate the ideas. (C) 2004 Elsevier Ltd. All rights reserved.