The Soviet Union had the most arduous of domestic championships. Players placing high in sundry “national” championships (of Georgia, Latvia, Russia, Armenia, et cetera) played semi-finals. Qualifiers from the semis played a 20-player round robin, with several world champions. Anybody who made the final league was at the least, a credible Candidate.
India had a similarly arduous championship, though the field was much weaker. Hopefuls played state championships and then a semi-final, the “National B”. The top 14 from the B qualified for the championship proper (alias the “National A”). The A was a round-robin featuring 20 players with the