The technology of optimal geometrically deformed tubes ("knitted tubes") for post-column derivatization in high-performance liquid chromatography is described. The three-dimensional wave shape induces a controlled Dean's flow that results in a volumetric dispersion that is very small and practically independent of the flow rate for the practical range of solvents and analytes. The three-dimensional wave consists of upward and downward turns that are offset from each other by 90 degrees to 120 degrees. Such a shape is commonly created by "knitting" or "weaving" Teflon (R) tubing. The strongly reduced dispersion compared to straight tubes or simple coils has made this type of device most popular in post-column derivatization. The hydro-dynamics of these devices have only been studied from the standpoint of the result. It is hoped that this publication will encourage further studies that achieve a deeper understanding of the underlying fluid flow. (C) 2009 Elsevier B.V. All rights reserved.