There is a letter in the unicode standard 'Modifier Letter Rhotic Hook' at $02DE. I was wondering what it is used for and who uses it ?
Which languages (if any) use this character and is it worth including in a font ?
I have looked on the internet but apart from it appearing in the unicode standard there seems to be very little information about it.
I suppose the main question is 'Is this character in wide useage ?'
For example, in most American English, a hard r is used in words like, well, hard. One exception being the accent typically associated with Boston. Most British accents tend to drop the r in these kinds of words (again, with regional exceptions).
The rhotic hook is added to the right side of the preceding vowel to indicate that, in a particular regional accent, the r that follows it is pronounced.
As for including it in a font, probably not, unless you also include all the other IPA diacritics.
I did some more research and IPA seems to define 28 vowel characters. In theory, each should be able to combine with the rhotic hook.
Most of these could be handled with a mark positioning feature to combine nicely, but the ones marked here in red need special attention (these include the two characters that are defined in Unicode). Those combinations could be replaced by a precomposed glyph when they occur.
FWIW, I had the pleasure of proof-reading this book. The main that thrust of the book is the final two chapters which recommend a way for linguists to store information about orthography profiles for languages they're working on, and the first six chapters feel like they're working up to that, but it's still worth a read. As a langscipress book it's open source and the whole thing is available on github at https://github.com/unicode-cookbook/cookbook
Compare the words «ban» and «bad» in American English; the vowels should be notably different. The former is nasalized and slightly raised.
This font family should be released next February (finally!). It includes a large phonetic set aimed to cover almost all linguist needs.
The image does not show the phonetic glyphs that are part of "regular" Latin script, like ɑ, nor combining or modifier glyphs.