Spring Festival couplets, is the first couplet on the left or the right? Why?

In ancient times, when posting Spring Festival couplets, the upper couplet was on the right, the lower couplet was on the left, and the order of banners was from right to left. However, after liberation, because the horizontal writing format was changed from left to right, the Spring Festival couplets were also changed to the upper couplet on the left and the lower couplet on the right, and the order of banners was also from left to right, which was more suitable for people to read, but it was not good to mix them together.

Spring Festival couplets are divided into upper couplets and lower couplets. It is not easy to distinguish the meaning of upper and lower couplets. Generally speaking, the first word of the first couplet is "Xu", which is three or four tones, and the first word of the second couplet is "Ping", which is one or two tones. According to this law, we can accurately distinguish the upper and lower couplets.