<qa>  What is the difference between the two 4-week randomised treatment periods?This is a 16-week, multi-centre, randomised, double-blind, double-dummy, crossover study of 4-week randomised treatment periods to demonstrate the efficacy and safety of 5 ug of Ba 679 BR inhalation solution from Respimat compared to tiotropium inhalation powder capsule 18 ug via HandiHaler in patients with COPD. The two 4-week randomised treatment periods are separated by 4-week washout period. 