Introduction

The interpretation of the diverse forms of observed radio source structure has always been problematical since this normally involves the use of some form of classification scheme. With the benefit of hindsight, it is clear that this exercise has not always proved to be a total success. Every astronomical object is the product of a unique set of physical circumstances which must, at some level, ultimately preclude the imposition of a generalised classification scheme covering many objects. It remains, however, a necessary basic stage in the process of scientific investigation. Any classification scheme is based upon gross structural features derived from observation. Such observations are of an inhomogeneous set of objects and are limited by sensitivity and imaging techniques. Schemes are therefore subject to strong selection effects and their subdivisions are arbitrary. A scheme can, however, prove useful provided the subdivisions broadly map out differing segments in the parameter space of the physical conditions of radio sources. The problem is, of course, that it is those very same physical conditions that are as yet unknown and that one is attempting to investigate. Thus, any current classification scheme is dominated by the characteristics of the telescopes available to observers at the time, and incorporates the ‘conventional wisdom’ derived from the interpretation of previous work. Such circumstances are profoundly inelegant but probably unavoidable.