I have read that the kun-reading is used when the kanji is used on it own.
And the on-reading used when the kanji is a part of a compound (two or more kanji characters are placed side by site)
For the kanji 一
The kun-reading is ひと
The on-reading is イチ,イツ
So can anyone explain to me why we used on-reading for the kanji 一 which is イチ when it comes by it Self not the kun-reading ひと.