It is said that there used to be 10 mountains including "Ichi ('一' meaning the number 'one')" in Narutaki and "I ('い,' the 2nd letters of hiragana)" in Ichihara that organized the okuribi (ceremonial bonfire) but, since the number of the mountains decreased to the present 5 after World War 2, this name began to be used.
かつては、鳴滝の「一」や市原の「い」など、送り火を行う山は十山あったと言われているが、戦後になって、現在の五山に減少した後に、この呼び名が生まれた。