1560: Proto-, Pre-, and Old in Practice Mar 23, 2019

In the field of historical linguistics, there are many terms to describe older forms, or hypothetical forms, of a language, but this is often as much convention as it is description. The qualifiers 'Proto-', 'Pre-' and 'Old' are often used to denote different stages of the development of languages, with the first referring to the oldest common ancestor of multiple languages, and the other two denoting older forms of a specific language, but with 'Old', as in 'Old English' more attestable. However, there is no exact science when it comes to predicting older languages, and sometimes these definitions don't totally hold, as in 'Old Norse' and Proto-Norse', which are both attestible. 
For more on Historical linguistics, watch this video: https://www.youtube.com/watch?v=E8wxfur1HwI&t=5s