All impressions printed at one time without alteration of the plate belong to the same issue. Thus, if two impressions are different states, the plate has been altered and they cannot belong to the same issue. However, an unaltered plate might have been used several times over a period years. In that case, the several issues would all be of the same state. Issues can sometimes be distinguished by the watermark, since different paper might have been used for each issue. For maps from atlases, different issues can often be distinguished by the text on the verso.