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Monopoly Rules and Variants
Monopoly: the official rules and unofficial variants.
Improving Monopoly
How to make your next game of Monopoly better than ever.
Rules - Stock Exchange
From Hasbro, a PDF version of the 1936 rules for Stock Exchange, a Monopoly add-on.
Rules - Various Editions
Use this page on Hasbro's web site to track down the rules for a variety of Monopoly editions.
Variants
From Jacob Davenport, a lengthy list of home rules from Bank Loans to Unlimited Housing. An excellent list.