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"metal," "silver," "copper" in Mallory, J. P. & Douglas Q. Adams. The Encyclopedia of Indo-European Culture. Fitzroy Dearborn Publishers. 1997.
Beekes, Robert S. P. Comparative Indo-European Linguistics: An Introduction. John Benjamins Publishing Co. 2nd Edition. 2011.
Lendinara, Patrizia. "The Survival of Indo-European Words in Old Frisian" in Aspects of Old Frisian Phonology. Ed. by Geart van der Meer, Rolf Bremmer Jr., & Oebele Vries. Rodopi. 2007.