General · EdgepediaSeptember 17, 20267 min read

Ø

Ø (lowercase ø) is a letter of the Danish, Norwegian, Faroese, and Southern Sámi alphabets, formed as an o with a diagonal stroke through it. It represents a close-mid front rounded vowel, the sound written [ø] in the International Phonetic Alphabet, and it occupies a place near the end of the alphabets that use it rather than being sorted under o.¹^•² In Unicode it is encoded as U+00D8 LATIN CAPITAL LETTER O WITH STROKE and U+00F8 LATIN SMALL LETTER O WITH STROKE, with the alias "o slash".¹

Key fact	Detail
Code points	U+00D8 (Ø) and U+00F8 (ø), Latin-1 Supplement block; Unicode name "O WITH STROKE", alias "o slash"¹
Alphabet position	28th letter of the Danish alphabet²
Origin	A ligature, a written combination, of the letters o and e; the combined form appears in ancient Latin inscriptions and survives in French œ (as in bœuf "beef")²
Languages	Danish, Norwegian, Faroese, and Southern Sámi; the lowercase ø is also an IPA symbol¹
Danish pronunciation	A rounded front vowel with three variants: half-high [ø], half-low [œ], and low [ɶ] in many r-words²
Distinct look-alikes	The empty set ∅ (U+2205) and the diameter sign ⌀ (U+2300) are separate code points, not the letter ø¹^•³

Origin as a ligature

The stroke-bearing letter grew out of a much older joined form. According to the Danish dictionary Lex, written by Allan Karker and the professor emeritus Chr. Gorm Tortzen of the Society for Danish Language and Literature, ø is a ligature of o and e, a form already seen in ancient Latin inscriptions and still used in French in the letter œ.² The Danish language portal sproget.dk adds that ø has been used in Danish writing since the 1100s, when Danish began to be written with Latin letters, and that medieval Nordic manuscripts used ø and oe as interchangeable variants that did not change a word's meaning.⁴

On Nordic ground two written forms developed side by side: the slashed ø, called "the struck-through ø" (Danish gennemstregede ø), and, by analogy with the written ä, the dotted forms ó or ö, called "dot-ø" (prikke-ø). Swedish, and later Icelandic, made ö the sole form, while Danish and Norwegian kept both in modern use, writing ó in handwriting and ø in print.² In the 1700s and 1800s the language scholars Jens Pedersen Høysgaard and Rasmus Rask tried, without success, to introduce ö for an open ø-sound in words such as rör in contrast to the closed vowel in mør.²

Danish practice tolerated the dotted form longer than one might expect. Until 1957, Danish topographic maps generally used ö in place names, for example Helsingör and Læsö, to prevent the stroke in ø from being mistaken for a line in the map image; later map series returned to the slashed ø.²

Sound values

In Danish the letter names the sound it represents. Lex describes Danish ø as a rounded front vowel with three variants: the half-high [ø], long as in søle or short as in sølle; the half-low [œ], occurring before m, n, and r and usually short as in øm, dømme, bøn, and skønne, though long in høne and skrøne; and a lowered [ɶ] in many r-combinations, long as in rør but usually short as in drøm and smør. The diphthong combinations øj and øg are pronounced [ʌj], as in høj and nøgle. Lex also notes that pronunciation cannot be concluded unambiguously from the spelling, comparing høre and chauffør with [ø] against gøre and valør with [œ].²

An older Danish grammar gives the same picture in more traditional terms: ø "has a sound like French eu in peu", for which English has no corresponding sound, in words such as Søster (sister), Sølv (silver), and Bøger (books). It notes that Danish spelling does not consistently distinguish the signs ö and ø, most writers using both promiscuously, with ö tending to appear before final m and n and before nn, rr, and rn.⁵ In foreign words, Danish ø stands for Latin oe and Greek oi, as in økonomi; in international transcription, Danish ø is rendered oe, as in Oeresund.² The lowercase ø is also the IPA symbol for the close-mid front rounded vowel itself.¹

Encoding and look-alike symbols

Unicode encodes the letter as a single character in the Latin-1 Supplement, where it sits among the accented and extended Latin capitals and lowercase pairs of that block.⁶ The code charts list related characters that differ from it: Ɵ (U+019F, o with middle tilde), Ꝋ/ꝋ (o with long stroke overlay), and, for the lowercase, the old Polish letter ꟁ (U+A7C1).¹

Three look-alikes cause most of the practical confusion. The empty set ∅ (U+2205), the diameter sign ⌀ (U+2300), and a slashed zero formed from digit zero plus a combining long stroke overlay are all visually close to ø yet distinct.¹^•³ The empty set has a direct historical connection to the letter: a 2015 Unicode proposal records that the introduction of the modern empty set symbol is attributed to André Weil of the Nicolas Bourbaki group, who, according to his autobiography, used the capital letter Ø of the Norwegian alphabet to create it; the symbol first appeared in Bourbaki's Théorie des ensembles, published in 1939.⁷ The proposal stresses that the slashed letter is not a negated letter O, "any more than the slashed letter Ø is a negated letter O".⁷

The diameter sign illustrates how the confusion runs the other way: in engineering drawings ⌀ precedes a measurement, as in ⌀20 mm, and because it is awkward to type, people often substitute ø or Ø, which reads fine to a person but is the wrong character.³

Ø as a stand-in for zero

Because a slashed zero distinguishes the digit 0 from the letter O, the slashed-o glyph has been pressed into service for zero as well. The United States Patent and Trademark Office's examination guidance notes the increasing use of the symbol ("Ø") as the standard representation of zero, the "slashed" or "canceled" zero being used to indicate zero and avoid confusion with the uppercase letter O in text and drawings. Since patent editors cannot perform technical review for each application, the Office resolved the ambiguity by an editorial rule: it prints the symbol Ø whenever the applicant uses that symbol.⁸

This use is contested on typographic grounds. Standardization efforts for handprinted characters in United States government work went the other direction: the committee's agreed rules kept the basic O shape and decided that the letter O, rather than the zero, should carry any extra identifying feature, on the reasoning that the zero appears more frequently in most applications and was therefore left simple.⁹ Writers on typography also caution that slashed zeros can be confused with the letter ø of Danish and Norwegian, and even with Greek phi, so the distinction the slash is meant to create can simply move one character over.¹⁰ A Washington State University usage guide adds practical objections: readers unfamiliar with the convention are confused by it, strings containing such characters may not sort properly, and a slashed zero created in one font can change to a plain zero in another.¹¹

Alphabetic status

Where a letter falls in the alphabet determines how words are sorted, and ø's placement differs from how other languages treat similar combinations. The Unicode core specification observes that most languages using the Latin script treat diacritically marked letters as variants of the base letter without independent alphabetic status, while some languages accord the combination full letter status; ø belongs to the second group.¹² Lex states the Danish position plainly: unlike international practice, where œ and ö are alphabetized under o, the letter is placed at the end of the alphabet in the Nordic languages.²

References

Topic: Encyclopedia › Society and history › Languages and linguistics › Writing and notation systems › Latin script

Initially written Sep 17, 2026 · Reviewed: Oct 25, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 25, 2026

Notice something wrong?

Ø