Introduction

Flat spectrum nuclei are found at the centre of many types of extragalactic radio source, including the powerful classical doubles, and the ‘isolated’ compact sources (that have comparatively weak extended structure). They turn out (when examined with sufficient resolution) to be the self-absorbed bases of jets that feed the extended structure, whose continuations are often seen on much larger scales. The systematic properties of VLBI jets have been reviewed frequently and can be summarised thus:

1. The jets are nearly always seen on only one side of the nucleus. ‘Counterjets’ have only been seen in a few sources (for example 3C 236, Schilizzi et al. 1988).

2. The jets contain bright ‘knots’ of emission that are often seen to move outwards, away from the base at speeds commonly in the region of 5 to 10 times the speed of light (Ho = 100 km s−1 Mpc−1). Although some apparently stationary knots have been observed, for example the outer component in 4C 39.25 (Shaffer & Marscher 1988), none has ever been observed to move inwards (Marcaide et al. 1985).

3. The speeds and trajectories of the individual knots do not in general vary greatly as the knots move out; it is difficult to place very severe limits on acceleration because of the limited accuracy of positional measurements and the relatively short distances over which the knots are observed to move. Significant changes in speed and direction have been detected in 3C 345 (Biretta et al 1986).

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