Unicode Demystified

Double Diacritics

One other interesting oddity is that some combining characters in Unicode attach to two characters. There's a tilde, for example, that is meant to be drawn over a pair of letters, like so:

These special characters, which are used in Tagalog and in the International Phonetic Alphabet, are treated as normal non-spacing marks. That is, they are stored after the first of the two characters they appear over and are just drawn so that they hang over whatever comes next.

For compatibility with some legacy encodings, the standard also includes pairs of combining characters that when drawn next to each other (i.e., when applied to succeeding ...