The Rigveda Samhita is the oldest extant Indic text.It is a collection of 1,028 Vedic Sanskrit hymns and 10,600 verses in all, organized into ten books (Sanskrit: mandalas). The hymns are dedicated toRigvedic deities. The books were composed by poets from different priestly groups over a period of several centuries from roughly the second half of the 2nd millennium BC (the early Vedic period), starting with the Punjab(Sapta Sindhu) region of the northwest Indian subcontinent. The Rigveda is structured based on clear principles – the Veda begins with a small book addressed to Agni, Indra and other gods, all arranged according to decreasing total number of hymns in each deity collection; for each deity series the hymns progress from longer to shorter ones; yet, the number of hymns per book increases; finally, the meter too is systematically arranged from jagati and tristubh to anustubh and gayatri as the text progresses.In terms of substance, the nature of hymns shift from praise of deities in early books to Nasadiya Sukta with questions such as, "what is the origin of the universe?, do even gods know the answer?", the virtue of Dāna (charity) in society, and other metaphysical issues in its hymns. There are similarities between the mythology, rituals and linguistics in Rigveda and those found in ancient central Asia, Iranian and Hindukush (Afghanistan) regions.