Is there any reason a lot of body parts use the Month/Moon radical?


腕、胸、お腹、肘、脇、肩 are all body parts, and their radical is 月. I wonder how that
came to be?




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.

