Referenced In

The Travels of Marco Polo
by Marco Polo, da Pisa Rusticiano

"…the road from Yung Ch’ang to T’eng Yueh is not the one indicated."

The Travels of Marco Polo
by Marco Polo, da Pisa Rusticiano

"and to-day the pass which cuts this territory in the direction of T’eng Yueh is called Nang-Sung-kwan."