Tags for a popular PubMed citation from Connote and CiteULike. The tag cloud or "Wordle" at the top of the figure shows the tags from both CiteULike and Connotea for the Science article "Tuned responses of astrocytes and their influence on hemodynamic signals in the visual cortex" (PubMed id 18566287). As the frequency scale at the bottom left indicates, the tags are sized based on the number of times they were used to describe the article. As the colour key at middle-right shows, the tags are divided into three, manually assigned categories: 'personal', 'non-MeSH', and 'MeSH overlap'. Personal tags are those, like 'kristina', that do not appear topical, 'non-MeSH' tags appear topical but do not match directly with any of the MeSH descriptors for the article (listed on the bottom-left), and the 'MeSH overlap' tags have matches within the MeSH descriptors assigned to the article.