Answer:
It should be noted that even when the same material is rubbed on itself, there will be a charge displacement (plastic wrap for instance), hinting at more complexity than the triboelectric series. I seem to remember reading that some polar materials can become induced electrets under these circumstances.
Explanation:
By convention, we call one type of charge “positive”, and the other type “negative.” For example, when glass is rubbed with silk, the glass becomes positively charged and the silk negatively charged.
