The layer widths and repeat spacing of long-period polytypes (LPPs) have been determined using synchrotron radiation source (SRS) X-ray diffraction topography (XRDT). This method has proved to be a powerful,tool in investigating the spatial extent of one-dimensional disorder (1DD), long-period polytypes (LPPs) and the boundaries of polytype layers in silicon carbide (SiC). The resulting neighbourhood coalescence models have confirmed the validity of the sandwich rule even in the limit of two arbitrarily long LPPs, as well as the unique nature of the 6H polytype. A significant empirical trend is reported here that relates the thickness of LPP layers to the periodicity of the repeat stacking sequence measured on the topographs. A good correlation between the data suggests that this behaviour is governed by a simple mathematical expression t = kN(n). Values for k and n have been determined that relate the polytype thickness (t in microns) to the number of hexagonal layers (N) in the polytype stacking repeat. These values can be used to prompt questions about the limits of polytypism and disorder in SiC. (C) 2004 Elsevier Ltd. All rights reserved.