One drum - Ichinotsudumi

...A string is attached to the drum body and worn around the player's neck. During the Nara period, the thin-waisted drums (saiyoko) used in Tang music were introduced to Japan and were called Ichi-ko, Nik-ko, San-ko, and Shi-ko in order of size. Ichi-ko, Nik-ko, and San-ko are called Kogaku drums and were used as a set in the early days....

*Some of the terminology explanations that mention "Ichi drum" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…鼓胴に紐をつけ，奏者の首からかける。奈良時代，日本に伝わった唐楽用の細腰鼓（さいようこ）を小さなものから順に壱鼓，二鼓，三鼓，四鼓といった。壱鼓，二鼓，三鼓は古楽鼓といわれ，初期には一組で用いられた。…

※「壱（一）鼓」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報