The Second Empire Style, also called the French Second Empire style, can be traced back to France, specifically to the reign of Napoleon III.

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Who Was Napoleon III?
Napoleon III, the nephew of Napoleon I, was the emperor of France from 1852 to 1870. Born in 1808 in Paris, France, he grew up in exile. He began his quest for to gain back the throne in 1832, writing to let his ideas be known to the people. He became the Emperor of the Second French Empire. He was the first President of France elected by a popular election.

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Napoleon III made modern the French banking system, improved the railway system, and made the French merchant marine one of the most important in the world. Napoleon III started the Franco-Prussian War (also called the Franco-German War). The French Third Republic was proclaimed in Paris, and Napoleon went into exile in England, he died there in 1873.

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About the Style
Much of Paris was rethought under Napoleon III with large avenues and striking monumental buildings to replace medieval structures. The change in Paris duringthe Second Empire style had a strong impact on building design in Europe and the United States. The prototype for Second Empire style is the Opera Garnier, in Paris, projected by Charles Garnier.

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The Second Empire Style In Europe
In Europe, this style is particularly evident in:
- Paris and Vienna, both of which were heavily influienced by this style in the late 19th century.

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- Rome also saw the employement of this style after the Risorgimento, and Bank of Italy by Gaetano Koch is an important example.

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- In Britain, this style can be seen in the Methodist Central Hall, in Westminster, projected by Edwin Alfred Rickards.

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- In Germany, most of the apartments and public buildings of the period, are examples, including the Reichstag building, in Berlin.

Image source: https://www.bundestag.de/besuche/architektur/reichstag
In United States And Canada
In the United States, buildings related to this style were the Old City Hall, in Boston and the State, War, and Navy Department Building, in Washington, D.C.

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Key features
The general characteristics to identify this style were:
- Mansard roofs;
- Entry porch with stoop;
- Marble fireplaces with arched openings;
- Tall arched windows with cornices;
- Columns were usually paired;
- Exterior veranda with balustrades.

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Info source: https://www.britannica.com/art/Second-Empire-style