Tam Điệp pass explained

20.1313°N 105.8582°WThe Tam Điệp pass (Sino-Vietnamese name: Đèo Tam Điệp, demotic folk name: Đèo Ba Dội) is the name of the three passes between ancient Thăng Long and Ninh Bình and Thanh Hóa provinces.^{[1] [2]}

Notes and References

1. Đức Thọ Ngô, Vạn Nguyên Nguyễn, Philippe Papin - Đồng Khánh địa dư chí - Volume 2 2003 - Page 1533 "Roads, Routes and Itineraries - A main road runs from the mountain of Tam Điệp in the province of Thanh Hoá, at the southern end of the province, passing the stage posts of Ninh Du and Ninh Ða, as far as the Thanh Quyết river. On the ..."
2. Administration Atlas of Vietnam. Vietnam Natural Resources and Environment Publishing. Hà Nội, 2013.