Usui Reiki Ryoho (臼井靈氣療法) — "Usui’s spiritual energy healing method" — is the name for the practice as Mikao Usui himself taught it in Japan from 1922. Every branch of Reiki traces back to it.
The original system
Usui developed the method after his retreat on Mount Kurama and spent his remaining years teaching it in Tokyo. The system combined hands-on practice, the five principles, meditation techniques such as Hatsurei-ho, and — at the higher levels — the symbols and attunements.
The Gakkai
Usui founded a society, the Usui Reiki Ryoho Gakkai, to preserve and continue the teaching. It has continued in Japan through a line of presidents since his death, and for many years kept a quiet, largely private existence — one reason the Japanese form was little known in the West until recently.
Character of the practice
Compared with some later Western branches, the original system leans more toward meditation and self-cultivation than fixed procedure. Intuitive approaches such as byosen and reiji-ho sit at its heart, with the formal hand positions treated as a foundation rather than a rulebook.
A living root
Although it is the oldest form, Usui Reiki Ryoho is not a museum piece. The Gakkai has continued quietly in Japan for a century, and recent decades have brought a renewed interest in the original Japanese approach among practitioners elsewhere, who look to it to rebalance what Western Reiki had gradually reshaped.
It is best thought of less as one school among many than as the root the others grow from. Whatever branch a practitioner follows, their practice can be traced back, in the end, to what Usui taught in Tokyo.