Because vowels can be pronounced in different ways and because the way a vowel is pronounced is not always apparent from looking at the word, especially in English, dictionaries and pronunciation guides use markings to indicate how a vowel should be pronounced. These marks are referred to as diacritical marks. In standard English words, these diacritical marks are not used when words are written out, but only used in reference sources as a guide to pronunciation.
The diacritical mark that indicates a vowel should be pronounced with a long sound is called the macron. The macron is represented as a short horizontal line placed directly over the vowel that should be pronounced with a long sound. For example, in a dictionary the word "ate" would be displayed with a macron above the "a" and the word "evil" would be displayed with a macron above the "e". The macron may also be used with the consonant "y" when it acts as a vowel such as in the word "cry".
The breve is the diacritical mark that indicates a vowel should be pronounced with a short vowel sound. The breve is a half moon shape that looks like a parenthesis printed sideways with the opening at the top. It would be placed over the "a" in "cat" or the "i" in "sit". It is used with all vowels. However, when "y" is used as a vowel with a short sound, it is printed as an "i" with a breve over it instead of a "y" with a breve.
In some words with multiple syllables some vowels may have an indistinct or unaccented sound. In this case a schwa is used to indicate the pronunciation of these vowels. This diacritical mark is represented by an upside down lower case "e". Examples of cases where a schwa would be used to indicate vowel pronunciation include the "o" in common or the "i" in legible.
Other Diacritical Marks
In languages other than English a variety of diacritical marks are used to show proper word pronunciation. Some of these symbols appear only in reference guides showing pronunciation, but others appear any time the word is printed. Those used to indicate vowel pronunciation include the acute accent, which is indicated by a diagonal line that slants from right to left; the caret, which looks like an upside down "v"; the diaeresis, which is represented by two small dots above a vowel like the "i" in naive; the grave accent, which slants from left to right; the Polish ogonek, which is a small dot placed beneath a vowel; the ring or volle, which is a hollow circle placed above a vowel in Scandinavian languages and the umlaut which are two dots above a vowel used most often in German words.