Line 21 may refer to: Closed Captioning EIA-608, a standard for closed captioning for NTSC TV broadcast A Railway Line In operation Line 21 (Guangzhou Metro), a metro line of the Guangzhou Metro in China Line 21 (Zürich), Switzerland Yangluo Line, a metro line of the Wuhan Metro in China Under Planning Line 21 (São Paulo Metro), a future metro line of the São Paulo metro Line 21 (Shanghai Metro), a future metro line of Shanghai Metro