Because Unicode contains a large number of precomposed characters there are multiple ways a character can be represented. For example letter ö can either be represented as
U+00F6 LATIN SMALL LETTER O WITH DIAERESIS
U+0061 LATIN SMALL LETTER O + U+0308 COMBINING DIAERESIS
By performing normalization first, Aspell will only see one of these representations. The exact form of normalization depends on the language. Give the choice of:
if the precomposed character is in the target character set, then (1), if both base and combining character is present, then (2), otherwise (3).
Unicode Normalization is now implemented in Aspell 0.60.