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tokens: there are some 1015 defining sources outside the central distribution that should be examined individually for possible contamination by the host galaxy or other anomalies. The lower left plot in Fig. 7 extending to mas shows more detail near the centre, but the distribution remains remarkably symmetric even at the highest resolution. The lower right plot in Fig. 7 is a close-up view of the very central region within mas in each coordinate and shows again the predominance of defining sources and the significant fraction of sources in the very inner zone at a few 0.1 mas from the centre. The very small bias in declination is now visible with more sources in the negative values of . The proportion of defining sources (blue squares) increases with resolution, but in all three plots the optical and radio positions are statistically in very good agreement for all three categories of sources despite the larger uncertainties in the radio data for the non-defining and especially the VCS-only sources. 4.1.2. Angular separations Table 1 gives the overall statistics of the angular separation () between the optical and radio positions, subdivided according to the type of ICRF source. The table shows that 94% of the optical positions are within 10 mas of the radio position of the associated ICRF2 source. This is quite remarkable given the limited time coverage and the numerous limitations of the astrometric solutions for Gaia DR1 as explained in Lindegren et al. (2016) . For the defining sources the percentage is even higher (98%). There is in fact a very consistent pattern in that the fraction of separations within a given limit () is always higher for the defining sources than for the non-VCS sources, which in turn have a higher fraction than the VCS-only sources. Since the distinction between the three categories is irrelevant for Gaia (cf. Fig. 4 ), this confirms the difference in the quality of the radio positions expressed by the quoted uncertainties in ICRF2. Table 1 Number of ICRF sources with angular separations between the optical and radio positions. Notes. In brackets are the percentages of sources in the group below the given limits. Fig. 8 Distribution of the angular distances between the Gaia solution and the ICRF2 positions. Left: the 2054 solutions closer than 10 mas to their radio position. Right: same for the 258 defining sources. Figure 8 shows the distribution of separations in graphical form. The difference between the whole set and the set of defining sources is even more conspicuous than for the coordinate differences. The median is 1.16 mas in one case and only 0.61 mas in the other. As mentioned earlier this is primarily due to the very different quality in the ICRF2 between the defining sources and the rest of the catalogue and confirms a well-known feature of the ICRF2 with fully independent observations. 4.2. Normalised differences In this section we discuss the positional differences scaled by their standard uncertainties. We