"Cut the Candle to the West Window" and "Cut the West Window ***" are both from Li Shangyin's poem "Night Rain Sends to the North": You asked about the return date but have not yet arrived, the night rain in Bashan swells the autumn pond. How can I cut off the candles from the west window and talk about the rainy night in Bashan?
As for why it is the west window instead of the east window and the south window, there have been discussions on the Internet, but there is no authoritative statement.
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In Tang poetry and Song lyrics, the word "West Window" appears frequently. On the contrary, the word "East Window", "South Window" and "North Window" are less common. I personally think it may be related to the living habits and architectural structure of the ancients. It is more common for the master room to live in the west room, so the west window is often faced. Just like Du Fu's "window contains the snow of Qianqiu on the Xiling Mountain" also refers to the west window.
I am afraid that only Li Shangyin knows the real standard answer. We are all guessing, haha!