A magnetotelluric (MT” sounding curve obtained at a given location may contain contributions from 1-D, 2-D, or 3-D geoelectric components, either singly or in combination. The modeling and interpretation of such data depend upon the ability to assess the degree of influence exerted by the three possible structural components. Six existing MT sounding curves provide case histories of the performance of both conventional and new methods for estimation of structural dimensions. For data from a single location, all the methods must be based on the horizontal rotation properties of the impedance tensor. Of the three conventional dimensional indicators considered, only one (skew) provides a degree of satisfactory performance for practical data. Three recently introduced, normalized dimensional weights appear to offer better performance. Solutions to the 1-D MT problem are central to the issue of providing dimensional constraints. The inverse theories established by Weidelt (1972) and Parker (1980) provide two tests that can be applied to establish the existence of 1-D solutions. The formalism in these theories provides the basis for a systematic method for investigating the dimensional properties of practical MT data.