Context: What is known in Iranian linguistic history as the "Middle Iranian" era is thought to begin around the 4th century BCE lasting through the 9th century. Linguistically the Middle Iranian languages are conventionally classified into two main groups, Western and Eastern.

Question: When did the age of Middle Iranian come to a close?
Answer: 9th century

Question: What are the two main divisions of Middle Iranian?
Answer: Western and Eastern

Question: What era began in the ninth century?
Answer: Unanswerable

Question: What language was divided into a northern and southern division?
Answer: Unanswerable