Buck, C., D. (1949). A dictionary of selected synonyms in the principal Indo–European languages: a contribution to the history of ideas. Chicago: University of Chicago Press.
Matasović, Ranko. Etymological Dictionary of Proto-Celtic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online: http://iedo.brillonline.nl.ludwig.lub.lu.se/dictionaries/content/proto-celtic/index.html
Geiriadur Prifysgol Cymru: A Dictionary of the Welsh Language. http://welsh-dictionary.ac.uk/gpc/gpc.html
Robert Farren, PIE culture words collection, 2017
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When the etymological tree is displayed in hierarchical style, the nodes can be repositioned manually only within their level.
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This map shows the etymological links in a geographic context.