The "Rigveda" is a collection of books, created by multiple authors from distant parts of ancient India. These authors represented different generations, and the mandalas 2 to 7 are the oldest while the mandalas 1 and 10 are relatively the youngest. Yet, the Vedic Sanskrit in these books of the "Rigveda" "hardly presents any dialectical diversity", states Louis Renou – an Indologist known for his scholarship of the Sanskrit literature and the "Rigveda" in particular. According to Renou, this implies that the Vedic Sanskrit language had a "set linguistic pattern" by the second half of the 2nd millennium BCE. Beyond the "Rigveda", the ancient literature in Vedic Sanskrit that has survived into the modern age include the "Samaveda", "Yajurveda", "Atharvaveda", along with the embedded and layered Vedic texts such as the Brahmanas, Aranyakas, and the early Upanishads. These Vedic documents reflect the dialects of Sanskrit found in the various parts of the northwestern, northern, and eastern Indian subcontinent.