The Capitol of Puerto Rico (Spanish: Capitolio de Puerto Rico), also known as Casa de las Leyes (House of Laws), and most commonly referred to as El Capitolio (The Capitol), is the seat of the Legislative Assembly, a bicameral legislature composed of a Senate and a House of Representatives, responsible for the legislative branch of the government of Puerto Rico. Located in San Juan Islet immediately outside the city walls of the Old San Juan historic quarter in the capital municipality of San Juan, the oceanfront, Neoclassical Beaux-Arts style, entirely white marble-covered edifice was constructed by Rafael Carmoega between 1921 and 1929 to resemble the ancient Roman Pantheon in Rome, using as inspiration the Low Memorial Library in New York City. It was added to the National Register of Historic Places in 1977.^[3][4][5]

Situated in the middle of San Juan Islet in the Puerta de Tierra historic district, the Capitol of Puerto Rico is about 1 mile (1.6 km) from both the La Fortaleza, seat of the executive branch, in the western end of the Islet in the Old San Juan historic quarter, and the Supreme Court Building, seat of the judicial branch, in the eastern end of the Islet in Puerta de Tierra.

Prior to the establishment of the Capitol, the Legislative Assembly used the Palacio de la Real Intendencia (Palace of the Royal Intendancy) in the Old San Juan historic quarter, which now houses the Department of State.^[6] The idea of constructing a capitol was first introduced by former Resident Commissioner Luis Muñoz Rivera in 1907, and was partly meant to demonstrate some degree of civilian rule in Puerto Rico, which at the time was already a territory of the United States.^[7] The project proposed by Muñoz Rivera assigned $3 million for the construction and established rules for a contest among architectures to decide which one would do the work.

In September 1907, Commissioner of the Interior Laurence Grahame sent the official request for architects through the press and magazines of United States. The specifications for the building were that it was supposed to have three principal installations: the Executive Council, the House of Delegates, and the Supreme Court. It was supposed to be simple, conservative, and appropriate to the archipelago’s weather conditions. A total of 139 architectural designs were submitted from the U.S., Cuba, Canada, France, Spain, and Puerto Rico.

The evaluation of the proposals was headed by William F. Willoughby, president of the Executive Council, José de Diego, Speaker of the House and represented by Luis Muñoz Rivera, José S. Quiñones, President of the Supreme Court, and Laurence Grahame, Secretary of the Interior of Puerto Rico. Three American architects were also asked to join the commission: E.B. Homerde, John E. Howe, and Bowen Bancroft Smith. Three projects were selected, belonging to Frank E. Perkins (New York City), James H. Ritchie and Lewis B. Abbot (Boston), and Henry L. Beadle (New York).^[8]

The proposal chosen was the one from Perkins, which presented a Neoclassical style with little ornamentation. The main halls were organized around a central patio, with the south façade leading to the lobby, which had a dome above, featuring a style similar to the Pantheon in Rome, including the oculus. To the right, there would be a session hall for the Executive Council, and to the left one for the House of Delegates. The Supreme Court would meet in a hall facing north.

However, Luis Muñoz Rivera and the local press preferred another project that was disqualified by the evaluating committee, because the architect had included a slogan in the blueprints, which was forbidden. The project belonged to a Puerto Rican architect called Carlos del Valle Zeno. Del Valle Zeno's project presented a Baroque style, with neoclassic orders.

Despite public opinion, Perkins was hired to make the construction blueprints for the Capitol, which were then auctioned. It was not until 1920 that the project was retaken when the Commissioner of the Interior, Guillermo Esteves, published a new design. The change came after a revision from the Department of Interior, which found Perkins' design to be inappropriate. Under the supervision of architect Adrian C. Finlayson, along with Francisco Roldán and Pedro de Castro, new blueprints were made using the Spanish Renaissance style.

Construction of the building eventually began in 1921 under the leadership of Antonio R. Barceló, President of the Senate. However, construction was halted after completion of the first level. When Finlayson got ill, he was replaced by Rafael Carmoega who, along with architects Joseph O'Kelly, William Schimmelpfennig, Harry Pembleton, Albert Nichols, Luis F. Pina, and Gonzalo Fernós Maldonado, made another design to finish the edifice.

The new design incorporated some of the elements from Perkins's original design, including the dome. The model for the central structure and the dome was that of the Low Memorial Library at Columbia University.^[9] Georgia Marble from the marble deposits in Georgia was used for the building.^[10]

The building was inaugurated on February 11, 1929, with the first meeting of the Legislative Assembly taking place three days later.

The immediate area surrounding the Capitol is known as the Distrito Capitalino (Capitol District), which is a disconnected building complex comprised of early- to mid-20th century structures of varying architectural styles, including Neoclassical, Mediterranean Revival, Art Deco, and International Style, belonging to the Capitol. These are centered around a landscaped park facing the rear façade of the Capitol in-between Juan Ponce de León Avenue and Paseo Covadonga (Covadonga Promenade) from Calle Esteves (Esteves Road) to Calle Gral. Contreras (Gral. Contreras Road). Containing various memorials, sculptures, and statues to individuals and events significant to Puerto Rican history, the park serves as a center of patriotism and civic virtues.^[11]

The Capitol has two annex buildings functioning as office space for legislators, discreetly located in the immediate east and west grounds of the Capitol. The Rafael Martínez Nadal Building serves as the Senate Annex on the eastern Senate side of the Capitol with offices for senators and two hearing rooms, and the Ernesto Ramos Antonini Building serves as the House Annex on the eastern House side of the Capitol with offices for representatives as well as hearing rooms.

These identical buildings were constructed in a Tropical Modern, International Style by the Toro Ferrer firm in 1955. Each two-story building has a rectangular plan and a flat roof, and their exterior features strip windows with aluminum sunshades, glass panes, and marble facing. All spaces are laid along an interior courtyard with landscaped areas, reflecting pools, and two entrance lobbies at each end.^[11]

Several buildings bordering the immediate grounds of the Capitol are also part of its complex. Located next to the Senate Annex, the Antonio R. Barceló Building, the seat of the former School of Tropical Medicine, houses the Legislative Library, as well as offices for the House and Senate, Legislative Services, Superintendent of the Capitol, and Resident Commissioner of Puerto Rico. Located to the direct south of the Capitol, the Luis A. Ferré Building houses offices for both chambers, as well as a 700 space parking garage.

Located to the southeast of the Capitol, the Baltazar Corrada del Río Building houses legislative and admistrative offices of the Senate, while the building of the Antiguo Cuartel de la Policia Insular (Old Headquarters of the Insular Police), the former home of the State Commission of Elections, is used to house House archives, as well as administrative offices.^[12][13] Located to the southwest of the Capitol, the Ramón Mellado Parsons Building houses Senate offices.

The frieze on the main façade (north) of the building facing Luis Muñoz Rivera Avenue is inscribed with the following text in Spanish by the first Governor of the Commonwealth of Puerto Rico, Luis Muñoz Marín:

The frieze on the rear façade (south) of the building facing the Juan Ponce de León Avenue is inscribed with the following Gettysburg Address text in English by the 16th president of the United States of America, Abraham Lincoln:

The Architecture and Construction Archives at the University of Puerto Rico (AACUPR) is the custodian of the Capitol of Puerto Rico Collection (1924-1926). Approximately eight cubic feet in size, the collection contains architectural drawings, photographs and textual documents. The original drawings, executed in ink over drawing cloth, were produced by the Department of the Interior under the supervision of Puerto Rican architect Rafael Carmoega. The 28 originals and 38 blue prints show plans, elevations, sections as well as electrical and plumbing distribution. There is also a proposal for the surrounding gardens by the architectural firm of Bennet, Parsons and Frost and several plates showing entries for the building competition. The collection was transferred to the AACUPR by the Capitol Superintendency in 1986.

• List of Legislative Assemblies of Puerto Rico
• List of state and territorial capitols in the United States

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