The first spelling of 北 
showed two people back-to- back. 北 originally meant back. It took the meaning of North. Why 北 became North? A thesis explains that when large houses were built, the large reception hall had to allow the owner to stand facing south and back to North, 坐北朝南.
Evolution of the spelling of 北 from left to right:
To avoid confusion of meaning, the meat key 月 has been added, which is found in many characters related to the body and limbs. Thus, 背 bèi was born, and it .
北 běi,north
背 bèi, back
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6 October 2020