腕、胸、お腹、肘、脇、肩 are all body parts, and their radical is 月. I wonder how that
came to be?
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kanji etymology radicals
Is there any reason a lot of body parts use the Month/Moon radical?
That is because the radical 「月」 originally comes from two different kanji --
「月」 ("moon") and 「肉」 ("flesh").

The two were originally treated as two completely different radicals but they
are now often taught/treated to be the same radical, which is the big source
of confusion today (even among us Japanese).

When you find the radical 「月」 in different kanji, some are meant to be the
original Moon-月 and others, the Flesh-月. The 月 part looks the exact same, of
course, which is why it confuses us.

Thus, the 「月」 used in the kanji denoting body parts is called 「 **にくづき** 」.
These include:

肩{かた} (shoulder), 肺{はい} (lung)、脳{のう} (brain), etc.

The 「月」 used in the other group of kanji is called 「 **つき** 」 or 「 **つきへん** 」.
Some examples are:

朝{あさ} (morning)、朏{みかづき} (crescent moon)、服{ふく} (clothes), etc. Some are clearly
moon-related, some not so much. Don't ask me why; I am no kanji expert.
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There are two radicals that take the form 月: one is 月, "moon", and the other
is a simplified "combining" form of the character [肉]{にく}, meaning "flesh". It
is the second one that appears in kanji representing parts of the body.
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The left side is actually a form of 肉, and in traditional dictionaries the
radical for these characters is 肉. It's called にくづき (from 肉{にく}＋月{つき}) because
it looks like 月 but is historically a form of 肉.

Because 肉 looks just like 月 in these characters, some less traditional
dictionaries list these characters under 月 instead, to make them easier to
look up. Still, if anyone asked me what the radical for 肘 is, I'd say 肉.

The radicals are a system for organizing characters into sections in
dictionaries, and most of the time the radical is the semantic portion of
semantic–phonetic compounds. However, if that were always the case, dictionary
makers would need a very large number of radicals; the 214 Kangxi radicals are
a compromise, and characters in this system can't always be classified under
their semantophores. And in fact, the 214 radicals aren't written in stone.
Different dictionaries over the years have used different sets of radicals. So
when a dictionary maker chooses to list these characters under 月, they're
compromising to make the characters easier to find, and they're not _wrong_ to
do so, but keep in mind that it's non-traditional and that it's historically a
form of 肉.
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The form that looks like 月 has several different origins, most of which are 肉
(flesh), 月 (moon), and 舟 (boat, ). Those shapes have been conflated during the
development until the 9th century. The most authoritative dictionary today,
Kangxi Dictionary, divides them in two, 肉-affiliated (にくづき) to radical 肉
(#130) and non-肉 to 月 (#74).

This is also what we're taught in the school, but some educational
dictionaries that don't follow Kangxi sorting would merge them into 月 for the
sake of convenience. In number, 肉 kanji are way much beyond true 月 and others,
so a practically better way may be identifying limited number of non-肉 kanji
out of 肉.

> 朝 期 望 朗 朔 朋 服 朕 朦 朧 & 有 (this 月 is actually 肉, but Kangxi treat it as 月)

would be a sufficient list of kanji with radical 月 in the daily life. Other
kanji you may see with 月 shape in left or bottom are 99% originated from 肉.

Strictly speaking, Kangxi Dictionary makes difference between 肉-derived 月 and
other 月 in form, but only Taiwan and Hong Kong's standards obey it.

[![enter image description
here](https://i.stack.imgur.com/SSS4l.png)](https://i.stack.imgur.com/SSS4l.png)  
(From left: China, Hong Kong, Taiwan, Japan, Korea)

See:

  * [Unihan Radical-stroke Index, 肉](http://www.unicode.org/cgi-bin/UnihanRSIndex.pl?minstrokes=0&maxstrokes=50&submit=Submit&radical=130) (All kanji currently in Unicode with radical 肉)
  * [Unihan Radical-stroke Index, 月](http://www.unicode.org/cgi-bin/UnihanRSIndex.pl?minstrokes=0&maxstrokes=50&submit=Submit&radical=74)
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