Yachiyo (八千代市 Yachiyo-shi) is a city located in northern Chiba Prefecture, Japan.

As of December 1, 2015 the city has an estimated population of 191,853 and a population density of 3,730 persons per km². The total area was 51.39 square kilometres (19.84 sq mi).

Yachiyo is located in northwestern Chiba Prefecture on the Shimōsa Plateau. The Shin River, 10 kilometers (6.2 mi) in length, flows through Yachiyo and forms the upper part of the Inba Discharge Channel. Pollution is problematic along the river. Phosphorus, potassium, and nitrogen drain from vegetable farms along the length of the river. A 19 kilometers (12 mi) walking path was built by the city of Yachiyo, and features a pedestrian suspension bridge with an observation platform

After the Meiji Restoration, the villages of Owada in Chiba District and Aso in Inba District, Chiba Prefecture were founded on April 1, 1889. However, much of the area of these villages was already under the control of the Imperial Japanese Army, who had established a training school in 1876 for infantry maneuvers. The school remained an active military installation until 1945. Yachiyo Town was created by the merger of Owada Town with Aso and Mutsu villages in 1954. Yachiyo was elevated to city status on January 1, 1967.

Chiba Prefecture (千葉県 Chiba-ken) is a prefecture of Japan located in the Kantō region and the Greater Tokyo Area. The sixth most populous prefecture, and 27th largest by land area, Chiba is on the east coast of Honshu and largely consists of the Bōsō Peninsula, which encloses the eastern side of Tokyo Bay. Its capital is Chiba City

he name of Chiba Prefecture in Japanese is formed from two kanji characters. The first, 千, means "thousand" and the second, 葉 means "leaves". The name first appears as an ancient kuni no miyatsuko, or regional command office, as the Chiba Kuni no Miyatsuko (千葉国造). The name was adopted by a branch of the Taira clan, which moved to the area in present-day Chiba City in the late Heian period. The branch of the Taira adopted the name and became the Chiba clan, and held strong influence over the area of the prefecture until the Azuchi-Momoyama period. The name "Chiba" was chosen for the prefecture at the time its creation in 1873 by the Assembly of Prefectural Governors (地方官会議 Chihō Kankai Kaigi), an early Meiji-period body of prefectural governors that met to decide the structure of local and regional administration in Japan.

The compound word ’’Keiyō’’ (京葉), which refers to the Tokyo-Chiba region, is formed from the second character in Tokyo (京), and the second character in Chiba (葉), which can also be pronounced “kei” and “yō” respectively. This compound is used in terms such as the Keiyō Line, Keiyō Road, Keiyō Rinkai Railway Rinkai Main Line, and the Keiyō Industrial Zone.

During the Occupation of Japan (1945–1952) Chiba Prefecture was controlled by American forces from the second floor of the prefectural capitol building in the city of Chiba. Numerous other cities in the prefecture, including Chōshi to the north and Tateyama to the south, were used as bases of the occupation. The rich agriculture areas across the prefecture protected the region somewhat from the level of food shortage and starvation immediately following the war. The immediate post-war period was characterized by carefully planned industrial expansion in the north of the prefecture and the significant increase of agricultural production after land reforms across the prefecture. The Keiyō Industrial Zone brought together smaller industrial areas along the entirety of the western coast of Chiba Prefecture, and the industrial zone became, and remains an important center of heavy industrial production and large-scale port facilities in Japan. Cities to the northeast of the prefecture in close proximity to Tokyo were connected by rail to the capitol, and became and remain bedroom communities to Tokyo. Narita International Airport began operation in 1978 in Narita after much protest to replace the overcrowded Tokyo International Airport (Haneda Airport). The majority of international air traffic enters Japan via Chiba Prefecture. Agriculture, specifically of rice, and also vegetables to serve the Greater Tokyo Metropolitan Area expanded greatly and became a source of income to the northeast and central areas of the prefecture. Agriculture expansion of the central and southern regions of the prefecture was in contrast to the depopulation of these areas as a significant part of the population moved to the northeast of the prefecture as a result of the urbanization of Japan, a process that continues into the 21st century.

The Greater Tokyo Area is the most populous metropolitan area in the world, consisting of the Kantō region of Japan, including the Tokyo Metropolis, as well as the prefecture of Yamanashi. In Japanese, it is referred to by various terms, one of the most common being National Capital Region (首都圏 Shuto-ken).

A 2016 United Nations estimate puts the total population at 38,000,000. It covers an area of approximately 13,500 km2 (5,200 mi2), giving it a population density of 2,642 person/km2. It is the second largest single metropolitan area in the world in terms of built-up or urban function landmass at 8,547 km2 (3,300 mi2), behind only New York City at 11,642 km2 (4,495 mi2).

The area has the largest metropolitan economy in the world, with a total GDP (nominal) of approximately $2 trillion (¥165 trillion) in 2008. According to research published by PricewaterhouseCoopers, the agglomeration of Tokyo had a total GDP of $1.5 trillion in 2008 (at purchasing power parity), ranking again as the largest urban agglomeration GDP in the world

The National Capital Region (首都圏 Shutoken) of Japan refers to the Greater Tokyo Area as defined by the National Capital Region Planning Act (首都圏整備法 Shutoken-seibi-hō) of 1956, which defines it as "Tokyo and its surrounding area declared by government ordinance." The government ordinance defined it as Tokyo and all six prefectures in the Kantō region plus Yamanashi Prefecture. While this includes all of Greater Tokyo, it also includes sparsely populated mountain areas as well as far-flung Bonin Islands which are administered under Tokyo.

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