Azevedo, Milton. Portuguese: A Linguistic Introduction. Cambridge University Press, 2005.
Michiel de Vaan. Etymological Dictionary of Latin and the other Italic Languages. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
2014-01-21, "ad", "ecce", "ille", "olle / ollus"
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This map shows the etymological links in a geographic context.