One theory states that Ang Mo Kio had two bridges of significance: an old wooden bridge over the Sungei Tongkang river, known as "pang kio" in Hokkien, and a newer concrete bridge on Cheng San Road over a tributary of the Kallang River. Since concrete in Hokkien is 红毛灰 "ang mo he" "Western ash", the bridge and consequently the area around it became known as 红毛桥 "ang mo kio", short for "concrete bridge". The area has also been historically referred to as Kou Teu Kio, meaning "nine bridges".