There's a strait which is called Øresund in Danish and Öresund in Swedish.
Looking at Latin Capital Letter O with Stroke, it has no decomposition rules.
Looking at Latin Capital Letter O with Diaeresis, it decomposes into Latin Capital Letter O and Combining Diaeresis.
Why is Stroke not a Combining Stroke? I can't find any such Combining Mark at all. In other words, why is "Ø" a base letter (sorry, it seems there's no better term based on this answer) when "Ö" is not?
Contrary to what the other answers have stated, the Unicode Standard does not actually care whether Ø or Ð or any other character is “fundamentally a different letter” or not, whatever that may even mean. Nor does it matter what purpose certain diacritical marks are used for. It’s all about typography and the capabilities of fonts and text engines.
From section 2.12 of The Unicode Standard, Version 15.0.0:
Most characters that one thinks of as being a letter “plus accent” have formal decompositions in the Unicode Standard. [...] There are, however, exceptions involving certain types of diacritics and other marks.
Based on the pattern for accented letters, implementers often also expect to encounter formal decompositions for characters which use various overlaid diacritics such as slashes and bars to form new Latin (or Cyrillic) letters. For example, one might expect a decomposition for U+00D8 LATIN CAPITAL LETTER O WITH STROKE involving U+0338 COMBINING LONG SOLIDUS OVERLAY. However, such decompositions involving overlaid diacritics are not formally defined in the Unicode Standard.
For historical and implementation reasons, there are no decompositions for characters with overlaid diacritics such as slashes and bars, nor for most diacritic hooks, swashes, tails, and other similar modifications to the graphic form of a base character. In such cases, the generic identification of the overlaid element is not specific enough to identify which part of the base glyph is to be overlaid.
Essentially, overlaid diacritics are too ambiguous in most situations to be of practical use. A sequence “capital D + overlaid short stroke” could result in Ð, with the stroke crossing the left side of the D, but the stroke could just as easily go over the right side, or the middle (like Ꟈ), or lie somewhere in the upper or lower parts of the glyph. And there’d be no way to differentiate any of these options.
There is also the problem that diacritics like this often need to be contorted to fit the shape of the base character. Consider all the different letters with an overlaid tilde for instance: ᵬ ᵭ ᵮ ᵯ ᵰ ᵱ ᵲ ᵳ ᵴ ᵵ ᵶ. Modern fonts can certainly deal with this, but any font that didn’t specifically account for all possible combinations would look awful, potentially even leaving some letters unidentifiable.
A similar issue exists for the palatalized hook and retroflex hook diacritics, which is why these are also not used for decompositions. For example, an s with a palatal hook looks like ᶊ which is easy enough, but an ʃ with a palatal hook actually has it attached to its side by an extra “stem” like this: ᶋ
There are very few overlaid diacritics in Unicode and those that are actively used mostly belong to writing systems where complex shaping rules and glyph substitutions are a prerequisite for rendering any text at all.
For the same reason that eth (Ðð) doesn't compose into Dd + COMBINING SHORT STROKE OVERLAY. Namely, it looks like a D with a stroke through it, but it's fundamentally a different letter, and there's no systematicity to how you would add bars to other letters to change their meaning.
The umlaut/diaeresis on the other hand is used on a bunch of different vowels, generally in a systematic way—you put an umlaut on a back vowel to make it front, producing ü ö ä from u o a, or you put a diaeresis on any vowel to indicate it's not a diphthong, producing ë ï and so on.
Though a COMBINING SHORT/LONG SOLIDUS OVERLAY (U+0337 and U+0338) was eventually added, Ø is fundamentally considered its own thing rather than an accented form of O, much like Đ and G are (despite originating as modifications of D and C with an extra stroke).
(It's worth noting that this argument was originally applied in general rather than for the purposes of any specific language. In Spanish, Ññ is treated as its own letter, and there's no system for applying a tilde to anything else. But in Portuguese, it's applied systematically to vowels. So the tilde gets treated as a diacritic rather than an inseparable part of the letter Ñ.)
o + U+0337 and O + U+0338, respectively.
Commented
Jul 19, 2023 at 9:30
a + U+030A. Meanwhile, of the three very transparent ligatures ⟨æ⟩, ⟨œ⟩ and ⟨ffi⟩, the first two do not decompose at all, while the third decomposes to f + f + i (with compatibility decomposition). There’s no reason why ⟨æ⟩ and ⟨œ⟩ should not be considered compatible with a/o + e.
Commented
Jul 19, 2023 at 9:54
d + U+0337 (combining solidus) and ⟨đ⟩ to d + U+0335 (short stroke). There’s even less reason why characters like ⟨ƀ ǥ ᵽ ħ⟩ do not decompose to b + U+0335 – but they don’t.
Commented
Jul 19, 2023 at 10:03
∂ combined with a stroke rather than d combined with a stroke
Diaeresis was historically used in a number of letter combinations (a,e,i,o,u and sometimes y, even ι and υ) in a large number of languages, but ø / Ø are used only in Danish, Norwegian, Faroese and S. Saami, and slash (distinct from bar) is not historically widely used. The precomposed letters are preserved in Unicode for legacy reasons, and having a dedicated Ø key was a reasonable solution in Scandinavia, whereas five or so precomposed umlauted keys would be impractical.
