The paper deals with two case-forms of East Baltic nominals, the dative and the instrumental dual. It summarises what is already known about these case-forms from Lithuanian and Latvian dialects as well as from Old Lithuanian sources. It is demonstrated that these data imply a particular reconstruction of the dual inflectional endings in these cases for Proto-Baltic. Subsequently, the paper shows that etymological matches of the reconstructed Proto-Baltic endings of the dative and instrumental dual may be attested in two further branches of Indo-European. Such endings are found in Gaulish, which is a member of the Celtic branch, and in Ancient Greek.