You are probably looking at the letter O in German, and sometimes the diacritic mark called the 'umlaut' is placed above the letter, like this: ö. The umlaut can be placed on any vowel, and affects the pronunciation of the vowel. The umlaut is very important in the German language, but this is not the only use of this mark. It is used in other languages, and there may be systems of phonetic pronunciation that use the umlaut to distinguish among various possible sounds that a vowel can represent.
In English, this diacritic is sometimes used to indicate something called diaeresis, meaning a pause or hiatus in the pronunciation of a word. In the word zoölogy, for example, you might initially be tempted to see the word as a simple 3-syllable word, ZOO-lo-gy. But it is not. The 'umlaut', or 'diaeresis' above the second letter o indicates that it is separated from the first letter o, so the word is pronounced more like zo-O-lo-gy. Zo rhymes roughly with toe, a -O- rhymes roughly with the o in olive.
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