KAGRA is a newly built gravitational-wave telescope, a laser interferometer
comprising arms with a length of 3\,km, located in Kamioka, Gifu, Japan. KAGRA
was constructed under the ground and it is operated using cryogenic mirrors
that help in reducing the seismic and thermal noise. Both technologies are
expected to provide directions for the future of gravitational-wave telescopes.
In 2019, KAGRA finished all installations with the designed configuration,
which we call the baseline KAGRA. In this occasion, we present an overview of
the baseline KAGRA from various viewpoints in a series of of articles. In this
article, we introduce the design configurations of KAGRA with its historical
background.