title_body: How can I explain the origin of the dual number in my Slavic-influenced East Nordic conlang? The language I am creating is derived from East Norse and heavily influenced by Slavic. It has a quite complex grammatical structure - for example, it has not only taken over the two additional cases from Russian but also developed several own ones.
  
  One unique aspect is the dual noun form, which is avaliable for describing things that usually occur in complementary pairs. There are dual-only nouns for things that only occur in the described pairs (such as a romantic couple) and dual forms of ordinary nouns in “natural” couple forms (a pair of shoes, the poles of a magnet, my eyes - not two houses or a couple of road signs). Some features are either identic to either singular and plural. A part of the pronouns - most notably all second-person pronouns - are however unique (somewhat similar to the dialect of Bavarian spoken in Munich).
  
  I am wondering how I can “trace back” such a development. According to my knowledge, (correct me if I am blatantly wrong) neither East Norse nor Old East Slavic had the aforementioned features, and just “inventing it by accident somewhere around 1100” would sound like a too handwaved explanation.
upvoted_answer: Both Proto-Germanic and Proto-Slavic had the dual grammatical number. So you could just say that your conlang retained it the whole time. Alternatively you could say that it lost it and then subsequently borrowed it again from one of the Slavic languages which retained it.
  
  If you were after a specific source, Old Church Slavonic could be ideal because it could have a wider influence than other Slavic languages through its religious prestige status. If the religious traditions of the Slavonic Orthodox churches were very highly valued by your conlang's speakers then that would help to explain why the dual number was retained by them even though the other languages of the region had lost it.