The tandem bicycle or twin is a form of bicycle designed to be ridden by more than one person.

Tandem Bicycle

The tandem bicycle or twin is a form of bicycle designed to be ridden by more than one person.

"A penetration twin, since the two individuals interpenetrate each other." — Ford, 1912

Twinned cubes

"A penetration twin, since the two individuals interpenetrate each other." — Ford, 1912

"A contact twin, since the two individuals lie simply in contact with each other upon a certain plane." — Ford, 1912

Twinned octahedron

"A contact twin, since the two individuals lie simply in contact with each other upon a certain plane."…

"An octahedron twinned on an octahedral face has the two portions symmetrical with repsect to a plane parallel to this face (the large triangular face in this figure)." -The Encyclopedia Britannica 1910

Spinel-twin

"An octahedron twinned on an octahedral face has the two portions symmetrical with repsect to a plane…

"Condensing Twin Prism Light.—Part of the Lamiash light in the Firth of Clyde. Its action will be easily understood by the numbers shown on the diagram." —The Encyclopedia Britannica, 1910

Twin Prism

"Condensing Twin Prism Light.—Part of the Lamiash light in the Firth of Clyde. Its action will…