Prefectures and regions 
Main islands of Japan are Hokkaido, Honshu, Shikoku, and Kyushu;
Honshu is the biggest islands. Japan
is divided into the Tokyo metropolitan area and 46
prefectures as administrative districts. It is also classified into
eight regions conventionally: in the order from the north, Hokkaido, Tohoku, Kanto,
Chubu, Kinki, Chubu, Shikoku, and Kyushu regions. In addition to the
regions, there are the Nansei Islands to the southwest of Kyushu and
the Izu-Bonin Islands to the south of Honshu (see right figure).
In this website, Japan is divided into seven regions for describing regional landforms and geology as follows: Hokkaido, northeastern Honshu (Tohoku), central Honshu (Kanto and Chubu), southwestern Honshu (Kinki and Chugoku) and Shikoku, Kyushu, the Nansei Islands, and the Izu-Bonin Islands.
We often use “northeast Japan” and “southwest Japan”. In terms of geology, the border between northeast Japan and southwest Japan is the Itoigawa-Shizuoka Tectonic Line running north and south in central Honshu.