Strings Name
The Koto has 13strings. These names are 1,2,3,4,5,6,7,8,9,10, 斗 (To), 為 (I), 巾 (Kin).
Why not 11,12,13? What are To, I, Kin?
Actually, before they were named by a number, they had been called by a word such as 仁(Jin), 智 (Chi), 礼 (Rei), 義 (Gi), 信 (Shin), 文 (Bun), 武 (Bu), 斐 (Hi), 蘭 (Ran), 商 (Shou), 斗 (To), 為 (I), 巾 (Kin).
Each Chinese character has meaning.
For example, 仁 (Jin) means “with warmth”, 智 (Chi) means “value of knowledge”, and so on.
When the Koto came to Japan from China in 8th century, it was used as one of the instruments of Gagaku which was the court music of ancient Japan.
We can see these old names if strings in the old Gagaku textbook “Kyokunsho” written by Koma Chikazane in 1233.
And Koto was called “仁智の器 (The instrumental of Jin Chi)” in an old Chinese book.
So the old names of strings might have been already named in China.
After that, Japanese people tried to replace a word with a number to make it easy.
From 1 to 10, they could write it in one simple Chinese character.
But over eleven, they needed two characters to write.
For example, 11 is written as “十一” . It would be confused whether it means 11 or means “十(10)” and “一 (1)”.
For that reason, they decided to keep it intact from eleven to thirteen.
It is easier for us to learn a number than a word.
On the other hand, the old name which means the teaching for good people shows us how carefully the ancient people treated the Koto and its music.
References:
http://www.kinkohdo.jp/middle/
http://www.kyogei.co.jp/shirabe/iroiro/a_koto.html
「箏入門の為の小品集」吉崎克彦著 大日本家庭音楽会 1990