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I noticed that Unicode has slots for the decomposition type of kanji. I have been wondering why it has only 12 slots (and doesn't even include a slot for "indecomposable", an empty box for a character like 木).

I can come up with more decomposition types, e.g. a long box on top and two small boxes on the bottom, for a character like 森.

Is it general practice to only distinguish the 12 types listed in Unicode? Is 森 analysed as being of something like "primary decomposition type" top (木) and bottom (林)?

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Sometimes if you are lucky you can find insights into Unicode's decision making process via their mailing lists. They're searchable with Google too. –  hippietrail Mar 2 at 19:18
    
@dainichi I'm asking about decomposition of Japanese kanji, i.e. whether 森 should be decomposed as "top, lower left, lower right" or as "top, bottom". How is this not about the Japanese language? A good answer needn't even necessarily mention Unicode. –  Earthliŋ Mar 3 at 1:45
    
OK, I retracted my close vote. I first read this as a question about Unicode, but I guess it's really about whether there is a "general practice", which the Unicode reflects. –  dainichi Mar 3 at 2:11
    
I'm PRETTY sure that 森 would be classified as 木 + 林, but not 100%. This has been your useless comment of the day. –  ssb Mar 3 at 3:33
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Everyone thinks about kanji of being made up from several components, but I've never come across a systematic way to decompose a kanji into its parts. It seems like Unicode has enough types to describe where the (unique type of) radical occurs. Maybe that's why there are only 12 types. –  Earthliŋ Mar 3 at 16:05

2 Answers 2

up vote 6 down vote accepted

The intended purpose of the Ideographic Description Characters is for describing characters that are not encoded in Unicode (see page 423 of the standard) in an Ideographic Description Sequence (IDS)---combinations of these characters and existing CJK characters.

At the time they were first introduced (Unicode 3.0) there were "only" ~27,000 CJK characters encoded (now there's over 75,000), and an IDS could be used to talk about CJK characters that did not exist in Unicode as compositions of those that did (which is why there is no marker for "indecomposable" and they are printable characters). That is, they are not intended to contain any particular semantic of etymological content (although they certainly can be used in that way). They are also not intended to be comprehensive---there are characters that cannot be decomposed because the required components do not exist (for example: 㣲, 莵, ...).

As far as Unicode is concerned, there is no canonical way of decomposing a character. The group behind them (the Ideographic Rapporteur Group) have made some suggestions about how to do so, and CHISE project has come up with a decomposition almost all of the existing CJK blocks in Unicode---but these are not rules or an established standard. So 森 could be ⿱木林 or ⿱木⿰木木 (but I think shorter sequences are preferred).

There are also some "missing" combinations. For example, there are surrounds for the top-right (⿺), bottom-left (⿹), and bottom-right (⿸), but not top-left. And similarly for surrounds above (⿵), below (⿶), and left (⿷), but not right.

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Thank you for this answer. It resolves a lot of my questions about decomposition of CJK characters. Reading the references you provide, I guess it is unlikely that there is a traditional or canonical way to decompose characters into its components. –  Earthliŋ Mar 21 at 2:02

While characters can be broken down into individual elements in a wide variety of ways, the Ideographic Description Characters in Unicode represent the most common, and in fact provide a substantial amount of coverage if one views deconstruction as a matter of recursion.

For example, 森 is 木+林 in a stacked configuration: ⿱. 林 is 木+木 side-by-side: ⿰. Nested within each other you effectively get your long box over two short boxes, but semantically that's not how the character works.

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