Guus Kroonen. Etymological Dictionary of Proto-Germanic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
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
De Vries, Jan (1977). Altnordisches etymologisches Wörterbuch. 3rd ed. Leiden: Brill
Robert Farren, PIE culture words collection, 2017
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This map shows the etymological links in a geographic context.