“…Firstly, unlike professional indexing systems, there are no restrictions or rules on how tags should be designed or applied: different users use different words for the same concept, homonyms are not disambiguated, hierarchical and other relationships between tags are often absent, tags may be written in different forms (singular/plural, spelling variations etc. ), they may be unlimited in quantity or may have relevance for personal use only (e.g., "to read") (Furner 2010, Gerolimos 2013, Golder and Huberman 2006, Guy and Tonkin 2006, Kipp et al 2015, Rolla 2009, Steele 2009. At the same time, they are characterized by the natural everyday language that the users are familiar with and can relate to, especially if compared to more formal and traditional subject indexing languages which may not always reflect current terms and may contain outdated terms (Adler 2009, Bates and Rowley 2011, Furner 2010, Spiteri 2006).…”