1. A method of identifying an article on a production line, the production line comprising a plurality of articles along its length, the relative positions of the plurality of articles along the length of the production line being predetermined, the method comprising: receiving location information of an object which executes the same motion with respect to at least a first subset of the plurality of articles on the production line; for at least a second subset of the plurality of articles, determining from the location information the location of the object at a predetermined interaction between the object and the article; and identifying an article of the plurality of articles at a specified location of the object in dependence on (i) a known location of any one article of the plurality of articles at any one time, and (ii) the locations of the object at the predetermined interactions between the object and the at least second subset of the plurality of articles.