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.
Articulated bus
A few attempts have been made to design a double-decker articulated bus. NEOPLAN Bus GmbH built a handful of Neoplan Jumbocruisers between 1975 and 1992. In these models, only the upper deck allows movement between the two sections, so each section has its own doors and set of stairs.