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Amongst the English vowels it sounds the most like the 'i' in "bird" [1] or the 'u' in "hurt" [2]. The name of the letter is the same as the sound it makes.

The origin of the letter is a ligature for the diphthong " o e" (the horizontal line of the "e" being written across the "o") that has become a letter in itself. In modern Danish and Norwegian, the letter is a unique vowel ( IPA [ø]), and neither a diphthong, a ligature, nor a variant of the letter "O". As one Norwegian tour guide put it, "It's not an 'O' with a slash, it's an 'Ø'."

In the Turkish, Finnish, Swedish, Icelandic, and German alphabets, the letter " Öor is a letter, representing a vowel, in the Finnish, Swedish, Icelandic, Estonian, Hungarian and Turkish alphabets. It also appears in the German alphabet, where it represents " O" with umlaut, and is alphabetized together with "O". In English, it design" is the equivalent.

In Danish (and more conservative, Danish-influenced Norwegian) spelling, ø is a word all by itself meaning island.

The symbol "Ø" is also used in mathematicsMathematics is commonly defined as the study of patterns of structure, change, and space; more informally, one might say it is the study of "figures and numbers". In the formalist view, it is the investigation of axiomatically defined abstract structures to refer to the empty setAbstract algebra Algebra Set theory In mathematics, the empty set is the set with no elements. Notation The standard notation for denoting the empty set, invented by Nicholas Bourbaki, is the symbol , also written as or ∅, and sometimes approximated, following Bourbaki. Modern typesetting software used by mathematicians typically renders it in stylised form. For example, common TeXDuane Bibby TX written as TeX in plain text, is a typesetting system written by Donald Knuth. It is popular in academia, especially in the mathematics, physics and computer science communities. It has largely displaced Unix troff, the other favored format packages offer \emptyset and \varnothing, which respectively appear as:

The symbol "ø" is also used in the International Phonetic Alphabet to indicate the sound of the Danish and Norwegian letter, a rounded close-mid front vowel. It is also used as the standard symbol for diameter, though the official symbol is slightly stylised (the stroke is often thinner at the bottom and thicker at the top, like the club or baton shape of the exclamation point; and extends further above the o portion). In German speaking countries it is also used as a symbol for average value: average in German is Durchschnitt, directly translated as cut-through.

For computers, when using the ISO 8859-1 or Unicode sets, the codes for 'Ø' and 'ø' are respectively 216 and 248, or D8 and F8 in hexadecimal. On the Apple Macintosh operating system it can by typed by pressing the [Option] key then typing O or o. On Microsoft Windows it can by typed by holding down the [Alt] key while typing 0216 or 0248 on the numeric keypad, provided the system uses code page 1252 as system default. The Unicode letter name is "Latin capital/small letter O with stroke". In HTML character entity references, required in cases where the letter is not available by ordinary coding, the codes are Ø and ø.

Ø is not to be confused with the number zero, which is sometimes written with a slash to differentiate it from 'O'. Once commonly used in this form by early computer models, recent computer technology has all but eliminated the need to use a slashed zero.