The island of Manhattan is home to the greatest concentration of private clubs in this country, and no, we are not talking about the velvet-roped doors of the Meatpacking District. These elite institutions date back to the early nineteenth century, modeled after the gentlemen's clubs of London and retain the codes of behavior from that era. The buildings are reverent but nondescript, the fees are astronomical, and the dress code is de rigueur. Here are 6 members-only clubs, where the upper echelon of New York Society can freely mix business with pleasure.