The purpose of this study is to evaluate the efficacy of this multicenter, randomized, double-blind, placebo-controlled, 4-period, incomplete-block crossover, dose-ranging study comparing 2 fixed dose combinations (FDCs) of aclidinium bromide with formoterol fumarate or with placebo, aclidinium bromide and formoterol fumarate, all administered twice a day (BID) in patients with stable, moderate to severe chronic obstructive pulmonary disease (COPD) beginning with a 2-week run-in period and with a 7-10 day washout each between treatment period.