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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.

In the orthorhombic system, tabular crystals of lead carbonite (cerussite), bounded by the forms ∞P∞̆, {010} (b); ∞P, {110} (m); and P, {111} (p), form groups in which both (m) and ∞P3̆, {130} act as twinning planes.

Cerussite

In the orthorhombic system, tabular crystals of lead carbonite (cerussite), bounded by the forms ∞P∞̆,…

"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

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example of malachite.

Malachite

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example…

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded by ∞ P, {110} (M); P∞̆, {011} (l); and 0P, {001}, and united into a pentagonal figure by (M) as twinning plane.

Marcasite

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded…

"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."…

This illustration shows a solid state where the orientation is only partial, as in the case of twin crystals. Here, the molecules of each horizontal row have two of their axes parallel with those above and below them, but not the third.

Partial Orientation of Molecules in a Solid

This illustration shows a solid state where the orientation is only partial, as in the case of twin…

The icositetrahedron, 4O4, {411}, on potassium chloride sometimes produces an apparent rhombohedron by the survival of only one fourth of its planes. Two such rhombohedrons, in apparent twinning position, may be derived from the planes of the same icositetrahedron.

Potassium Chloride

The icositetrahedron, 4O4, {411}, on potassium chloride sometimes produces an apparent rhombohedron…

"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…

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated in the case of the triclinic feldspar, Albite. The twinning plane here is the brachypinacoid, and the contact of a large number of fine lamellæ, alternately in twinning position, would evidently produce a striation on the basal plane parallel to the brachydiagonal axis

Striation of Crystal Planes

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated…

"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…