A PID or persistent identifier has been in common use in scientific publishing for around 20 years now. It was introduced as a DOI (Digital Object Identifier), and the digital object in this case was the journal article. From 2000 onwards, DOIs started appearing for most journal articles, journals having obtained them from a registration agency, CrossRef. This is a not-for-profit organisation set up by a publishers association for the purpose. Most readers of journal articles started to use this DOI as an easier way of navigating through invariably different and sometimes confusing metaphors set up by any given journal to navigate through its issues. Readers slowly learnt to prepend the URL http://dx.doi.org/ to the DOI to “resolve” it directly to what is known as the “landing page” of the article. More recently, the prefix recommendation has changed to the slightly shorter https://doi.org/ form. Few readers are aware however that the DOI can serve a much more interesting purpose than just taking you to the article landing page. This post will explore a few of these extras.