Article: Czech has one of the most phonemic orthographies of all European languages. Its thirty-one graphemes represent thirty sounds (in most dialects, i and y have the same sound), and it contains only one digraph: ch, which follows h in the alphabet. As a result, some of its characters have been used by phonologists to denote corresponding sounds in other languages. The characters q, w and x appear only in foreign words. The háček (ˇ) is used with certain letters to form new characters: š, ž, and č, as well as ň, ě, ř, ť, and ď (the latter five uncommon outside Czech). The last two letters are sometimes written with a comma above (ʼ, an abbreviated háček) because of their height. The character ó exists only in loanwords and onomatopoeia.

Question: What have some of Czech's characters been used for by phonologists to denote?
corresponding sounds in other languages