We have learnt 乎 before and now we are going to examine its close relatives: 兮. From their modern form, we probably would not be able to tell how close they are, but if we compare their Oracle scripts, it will become obvious.
Oracle script of 乎: 
Oracle script of 兮:
of is to represent air flow that produces load sound. Now comparing 
with , we can tell that should be drawn to represent a sound that is lower than .Before we move on to conclusion, let us examine 兮's evolution history:
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| evolution history of 兮 |
兮 means to produce a lower sound than 乎, probably a sound at the level of a sigh. normally used as a modal particle per Chinese grammar, like 乎.
That is all for 兮, time to enjoy a Chinese calligraphy with it inside:
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| 归去来辞：归去来兮，田园将芜胡不归，既自以。。。 |
