A ward or ku is an administrative division used in major cities in Japan. All Japanese cities with populations over 800,000 are divided into wards, and there are currently thirteen such cities: Tokyo, Yokohama, Saitama, Chiba, Osaka, Kyoto, Kobe, Sapporo, Sendai, Nagoya, Hiroshima, Kitakyushu, and Fukuoka. Some of these wards are tiny, covering only two or three square miles: some cover scores.

Wards are these cities' administrative nerve centers. The ward offices are responsible for tax collection, health insurance claims, personal seal registration, and the supervision of resident aliens, among other things. While this same structure is found in other countries (South Korea's system is almost identical, partly owing to its history in the Japanese Empire), the closest American comparison I can think of is to the five boroughs of New York City.

Wards are also seen on Japanese addresses. For instance, NTT's headquarters are located at "3-1 Otemachi 2-chome, Chiyoda-ku, Tokyo." The address breaks down as block number, house number, district name and chome, ward, and city. To find a building from its address, you have to look at a map of the ward first, and then find the chome and block numbers from that map.