Standard colloquial Bengali — the chalit bhasha that replaced the Sanskritised sadhu bhasha in print in the early twentieth century, and the register in which Bengali is now taught, broadcast and written across two countries — descends from the Rarhi group of dialects, and specifically from the prestige speech of the Nadia–Shantipur tract at that group's eastern edge. That matters for two reasons. It means the grammar spoken by something over two hundred million people is this region's grammar and not the delta's. And it means the Rarh's own western varieties, in Bankura and Purulia, are heard in Kolkata as rustic while belonging to the same dialect group as the standard — the prestige attached to one end of a continuum and withheld from the other. West of that continuum the language changes family entirely: Kurmali is Indo-Aryan of the Bihari group, and Santali is Munda, part of the Austroasiatic family, related to nothing else spoken nearby.
Rarhi BengaliIndo-Aryan, Bengali–Assamese
Spoken across Bardhaman, Birbhum, Bankura, Hooghly and Howrah, and eastward into Nadia. Its Nadia–Shantipur variety is the historical basis of the modern written and spoken standard.
Speakers: The dialect group underlying standard Bengali
ManbhumiIndo-Aryan, Bengali–Assamese
The Purulia and western Bankura variety, with heavy Kurmali and Santali contact influence in vocabulary and intonation. It carries the Jhumur, Tusu and Bhadu repertoires.
KurmaliIndo-Aryan, Bihari group
Spoken across the Purulia–Jharkhand fringe by Kurmi and neighbouring communities, and used alongside Bengali rather than instead of it. There is a long-standing demand for its inclusion in the Eighth Schedule.
SantaliAustroasiatic, Munda
A wholly different language family from everything around it, with its own script — Ol Chiki, devised by Raghunath Murmu in the 1920s — and included in the Eighth Schedule of the Constitution in 2003. In this region it is spoken in villages that are otherwise indistinguishable from Bengali ones.
Speakers: Around 73 lakh in the 2011 Census across the eastern states
Bhumij and Mundari on the far edgeAustroasiatic, Munda
Present in the western Purulia villages and shading into the Jharkhand and Odisha plateau, with Bhumij now largely bilingual in Bengali.