Double-decker bus
The first commercial horse-drawn double-decker omnibuses were introduced in England in 1847 by Adams & Co. of Fairfield, Bow, then improved upon by John Greenwood, who introduced a new double-decker in 1852.
Open top bus
Until the 1920s most, if not all, double-decker buses were constructed with no roof on the upper deck, and were the original "open-toppers".