Obelisks were built in order to honor or commemorate important people or events in history. Here's a list of the most notable obelisks from around the world.
Cleopatra's Needles

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Cleopatra's Needle from the River Thames, London

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Cleopatra's Needle in Paris at the Place de la Concorde

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Cleopatra's Needle, Central Park, New York
Three of the most popular obelisks in the world are the so-called Cleopatra's Needles. They are a trio of obelisks in London, Paris, and New York City. Each is made of red granite, stands about 21 meters (68 ft) high, weighs about 180 tons and is inscribed with Egyptian hieroglyphics. They were originally erected in the Egyptian city of Heliopolis on the orders of Thutmose III, around 1450 BC. The inscriptions were added about 200 years later by Ramesses II to commemorate his military victories. The obelisks were moved to Alexandria and set up in the Caesarium - temples built by Cleopatra in honor of Mark Antony - by the Romans in 12 BC, during the reign of Augustus, but were toppled some time later. This had the fortuitous effect of burying their faces and so preserving most of the hieroglyphs from the effects of weathering.
Obelisk of Pharaoh: Egypt

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Ancient Egypt had built numerous obelisks but most of them were either destroyed or transported elsewhere in the world. One of the few that remained is the Obelisk of Pharaoh Senusret I. It is located in Al-Masalla area of al-Matariyyah district in Heliopolis, Cairo.
Obelisk of Thutmosis III: Turkey

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The Obelisk of Thutmosis III has large hieroglyphics that can be seen at a distance. It is currently standing in Istanbul, Turkey.
King Ezana's Stele: Ethiopia

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The structure known as King Ezana's Stele is the central obelisk still standing in the Northern Stelae Park in the ancient city of Axum, in modern-day Ethiopia. The obelisk was carved and erected in the 4th century by builders of the ancient Kingdom of Aksum. It is named after King Ezana, the first monarch of Axum to embrace Christianity. At its base, it is decorated with a false door, and apertures resembling windows on all sides.
King Ezana's Stele stands 70 ft (21 m) tall, smaller than the fallen 108-foot (33 m) Great Stele and the better-known 79-foot (24 m) Obelisk of Axum; this obelisk was reassembled and unveiled on September 4, 2008.
Washington Monument: USA

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This spectacular obelisk is called the Washington Monument. It is a large, tall, sand-colored obelisk near the west end of the national mall in Washington D.C. It is constructed to commemorate George Washington. The monument is the world's tallest stone structure, and is the world's second tallest obelisk (behind San Jacinto Monument, in Texas), standing 169.294 meters (555 ft 5⅛ inches) in height and made of marble, granite, and sandstone. The Washington Monument reflection can be seen in the aptly named Reflecting Pool, a rectangular pool extending to the west toward the Lincoln Memorial.
Obelisk of Sao Paulo: Brazil

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Obelisk of Sao Paulo is an obelisk in Ibirapuera Park in the city of Sao Paulo, Brazil. This monument is a symbol of the Constitutionalist Revolution of 1932, and the biggest monument of the city of Sao Paulo. The height of the monument is 72 meters. The construction of the monument was started in 1947 and completed in 1970. The obelisk, made of pure travertine marble, was inaugurated in 1955, one year after the inauguration of Ibirapuera Park.
One Tree Hill: New Zealand

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One remarkable obelisk in New Zealand is the One Tree Hill. One Tree Hill or Maungakiekie in Maori is a 182 meter volcanic peak located in Auckland, New Zealand. It is an important memorial place for both Maori and other New Zealanders. The suburb around the base of the hill is also called One Tree Hill. The hill's scoria cones erupted 20,000 - 30,000 years ago, creating lava flows that covered an area of 20 square kilometers. The summit provides views across the Auckland area, and allows visitors to see both of Auckland's harbors.
Obelisk of Buenos Aires: Argentina

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The magnificent obelisk of Buenos Aires was built in May 1936 to commemorate the 400th anniversary of the first founding of the city. It is located in the center of the Plaza de la Republica (Republic Square), the spot where the Argentine flag was flown for the first time in Buenos Aires, at the intersection of Nueve de Julio and Corrientes avenues. Its total height is 67 meters (220 ft) and its base area is 49 square meters (530 square feet). It was designed by architect Alberto Prebisch, and its construction took barely four weeks.