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For each peculiar galaxy an x-y position in millimeters was recorded, as measured from the lower-left (south-east) corner of each plate. These cartesian co-ordinates have been preserved, and are given with the description of each object in Volume I, along with the plate and field to which they pertain. These co-ordinates should be sufficient to locate the object to within a few millimeters. To locate the same objects on the ESO Quick Blue Survey films, it is necessary to subtract approximately thirty-five millimeters from both x and y.

Because of uncertainties in the exact centering of the plate at the telescope and in our measuring devices, it was not possible to produce precise right ascension and declination co-ordinates from the x-y co-ordinates directly. Because rotations as well as translations of the plates with respect to the sky were obviously present in some of the earliest plates in the survey, (but not in later plates), a single off-set could not be determined for all of the objects on a given plate. Instead, individual scaled overlays were prepared for every object in the Catalogue after rough positions had been derived from the millimeter positions on the plate. Each object was then re-examined and corrections to the nominal positions were measured to an accuracy of fifteen arc seconds. The final positions are given to the nearest second of time and tenth of a minute of arc. Although these positions still carry a somewhat optimistic appraisal of the errors, they are certainly an order of magnitude better than any derived from the original x-y positions alone.

In some cases the co-ordinates are over-specified. That is, because of the lack of symmetry of some objects, the ``centre'' is rather subjective. Additionally, because of the extent of some systems or because more than one major object is involved in a system, some visually averaged centre (such as the mid-point between an interacting double) was chosen which does not necessarily contain any member of the system. This type of ``centre'' was restricted to systems less than a few arc minutes in extent. For larger systems, the co-ordinates for individual members are usually presented. Time constraints precluded this in some instances, and so readers are strongly advised to interpret the listed co-ordinates with care, especially when dealing with multiple systems.