From a Ponte Vecchio-style bridge across the Thames to a monorail running through Regent Street, we take a tour of the London that never was through the centuries.

Amsterdam could have had a Parisian-style boulevard.

Around the turn of the last century, the city council accepted proposals for a new commodity exchange. It initially favored a design sponsored by hotelier W.P. Werker, who would have demolished a whole street of buildings between the Dutch capital’s central railway station and the Royal Palace on the Dam Square to create something of a miniature Champs-Élysées.

A monumental elephant in place of the Arc de Triomphe. An aerodrome in the Jardins de Bagatelle. Multiple Eiffel Towers. Take our tour of the Paris that never was!

By the middle of the nineteenth century, Barcelona was bursting at the seams. The city hadn’t expanded beyond its medieval walls, but its population had grown almost 50 percent between 1800 and 1850. The congestion was contributing to outbreaks of disease. There was clearly a need for expansion, but it wasn’t until 1853 that the central government in Madrid allowed Barcelona to tear down its walls.

Two expansion plans were introduced, one by Antoni Rovira i Trias, which was favored by the Barcelona city council, and another by Ildefonso Cerdá, which was favored by Madrid. Neither was implemented in full, but Cerdá’s, with its distinctive hexagonal blocks, proved by far the most influential.

Elevated railways, sky bridges, rooftop airports and a plan to drain the East River. New York City would have looked very different if these architects and engineers had had their way.

Various proposals have been made through the years for buildings and building expansions in America’s capital that came to naught — from a Lincoln Memorial in the shape of a pyramid to a palatial Executive Mansion on Meridian Hill.