Does anyone know what "IG i" means in the footnotes of an article?

I need to sketch the author's argument and identify his primary sources via the footnotes. Only, I have no idea what the footnotes themselves are! It says "IG i3", or something else, but it keeps using "IG." There is nothing in brackets anywhere in the article that identifies what... show more I need to sketch the author's argument and identify his primary sources via the footnotes. Only, I have no idea what the footnotes themselves are! It says "IG i3", or something else, but it keeps using "IG." There is nothing in brackets anywhere in the article that identifies what "IG" is. The article is "State and Religion in Athenian Inscriptions" (2009), writing by P. J. Rhodes.
Does anyone have a clue as to what IG is? Is it some sort of work that is assumed to be known by all historians? I'm a business major, so I wouldn't know :P.
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