Another type of reference relation which is not strictly textual is that of co-reference. An example of a chain of co-referential items is Mrs Thatcher ®The Prime Minister ® The Iron Lady ® Maggie. Hoey points out that co-reference ‘is not strictly a linguistic feature at all but a matter of real-world knowledge’. It is, of course, true that recognizing a link between Mrs Thatcher and The Iron Lady, for instance, depends on knowledge of the world rather than on textual competence. Halliday and Hasan suggest a continuum of cohesive elements that may be used for referring back to an entity already mentioned in the discourse. This continuum stretches from full repetition at one end of the scale to pronominal reference at the other. The following example is adapted from Halliday and Hasan to illustrate the point: