In 27 BC, when Augustus established rule by emperors, the provinces of the Roman Empire were divided into imperial provinces and senatorial provinces. Augustus professed that the senate would keep the finest portion of the empire while he would take on the hardship and danger of defending precarious provinces which were vulnerable to internal rebellions or to external attacks (in the case of provinces along the borders of the empire). In reality he kept the provinces where the bulk of the legions were stationed and left the senators provinces which would be unarmed and unprepared for battle. Among the senatorial provinces, Asia and Africa were assigned to ex-consuls, and the others to ex-praetors. It was established that only this class of senators could pronounce a death sentence. The propraetors chose their provincial assessors from their peers or from their inferiors. The proconsuls chose three assessors form those of equal rank, subject to the emperor's approval. In the imperial provinces which had more than one legion, the governors, the legati Augusti pro praetore, were lieutenants of the emperor appointed by him and were usually propraetors, although sometimes they were ex-quaestors or men who had held other offices below the praetorship. With the propraetors in the imperial provinces being subordinates of the emperor, the latter could exercise better control over the administration of their governors. Many of the letters written by Pliny the Younger have survived. Book 10 of this collection contains the correspondence he had with the emperor Trajan during his governorship in Bithynia et Pontus in 110–113. In these letters Pliny informed the emperor about affairs in his province and often asked for instructions on specific matters. Trajan provided them in his replies.