Ring (diacritic)
A ring diacritic may appear above or below letters. It may be combined with some letters of the extended Latin alphabets in various contexts.

Rings
Distinct letter
The character Å (å) is derived from an A with a ring. It is a distinct letter in the Danish, Norwegian, Swedish, Finnish, Walloon, and Chamorro alphabets. For example, the 29-letter Swedish alphabet begins with the basic 26 Latin letters and ends with the three letters Å, Ä, and Ö.

Overring
The character Ů (ů; a Latin U with overring, or kroužek in Czech Republic) is a grapheme in Czech preserved for historic reasons, which identifies a vowel shift. For example, the word for "horse" used to be written kóň, which evolved, along with pronunciation, into kuoň. Ultimately, the vowel [o] disappeared completely, and the uo evolved into ů, modern form kůň. The letter ů now has the same pronunciation as the letter ú (long [uː]), but changes to a short o when a word is morphed (e.g. nom. kůň → gen. koně, nom. dům → gen. domu), thus showing the historical evolution of the language. Ů cannot occur in initial position, while ú occurs almost exclusively in initial position or at the beginning of a word root in a compound. These characters are used also in Steuer's Silesian alphabet. The [uo] pronunciation has prevailed in some Moravian dialects, as well as in Slovak, which uses the letter ô instead of ů.
The ring is used in some dialects of Emilian and Romagnol to distinguish the sound /ʌ/ (å) from /a/ (a).
ů was used in Old Lithuanian in Lithuania Minor from the 16th till the beginning of the 20th century and for a shorter time in 16th-century Lithuania Major for diphthong [uo].
The ring was used in the Lithuanian Cyrillic alphabet promoted by Russian authorities in the last quarter of the 19th century with the letter У̊ / у̊ used to represent the /wɔ/ diphthong (now written uo in Lithuanian orthography).
ẘ and ẙ are used in the ISO 233 romanization of the Arabic alphabet. A fatḥah followed by the letter ⟨ﻭ⟩ (wāw) with a sukūn (ـَوْ) is romanized as aẘ. A fatḥah followed by the letter ⟨ﻱ⟩ (yā’) with a sukūn over it (ـَيْ) is romanized as aẙ.
Ring upon e (e̊) is used by certain dialectologists of Walloon (especially Jean-Jacques Gaziaux) to note the /ə/ vowel typically replacing /i/ and /y/ in the Brabant province central Walloon dialects. The difficulty of type-writing it has led some writers to prefer ë for the same sound.
Many more characters can be created in Unicode using the combining character U+030A ◌̊ COMBINING RING ABOVE, including the above-mentioned у̊ (Cyrillic у with overring) or ń̊ (n with acute and overring).
The standalone (spacing) symbol is U+02DA ˚ RING ABOVE.
The unrelated, but nearly identical degree symbol is U+00B0 ° DEGREE SIGN.
Although similar in appearance, it is not to be confused with the Japanese handakuten (U+309A ◌゚ COMBINING KATAKANA-HIRAGANA SEMI-VOICED SOUND MARK), a diacritic used with the kana for syllables starting with h to indicate that they should instead be pronounced with [p]. In Japanese dialectology, handakuten is used with kana for syllables starting with k to indicate their consonant is [ŋ], with syllables starting with r to indicate their consonant is l though this does not change the pronunciation, with kana u to indicate its morph into kana n, and with kana i to indicate the vowel is to be said as [ɨ].
In Canadian Aboriginal Syllabics, there are two ring characters: ᐤ (Cree and Ojibwe final w, or Sayisi o) and ᣞ (Cree and Ojibwe final w or final y).  This second smaller ring can combine as a diacritic ring above in Moose Cree and Moose-Cree influenced Ojibwe as a final y;  in Inuktitut, the ring above the /_i/ character turns it into a /_aai/ character.  In Western Cree, /_w_w/ sequence is represented as ᐝ.
Unicode has:

U+030A ◌̊ COMBINING RING ABOVE
U+00C5 Å LATIN CAPITAL LETTER A WITH RING ABOVE
U+00E5 å LATIN SMALL LETTER A WITH RING ABOVE
U+016E Ů LATIN CAPITAL LETTER U WITH RING ABOVE
U+016F ů LATIN SMALL LETTER U WITH RING ABOVE
U+1E98 ẘ LATIN SMALL LETTER W WITH RING ABOVE
U+1E99 ẙ LATIN SMALL LETTER Y WITH RING ABOVE
U+212B Å ANGSTROM SIGN
U+1402 ᐂ CANADIAN SYLLABICS AAIU+1430 ᐰ CANADIAN SYLLABICS PAAI
U+144D ᑍ CANADIAN SYLLABICS TAAI
U+146C ᑬ CANADIAN SYLLABICS KAAI
U+148A ᒊ CANADIAN SYLLABICS CAAI
U+14A4 ᒤ CANADIAN SYLLABICS MAAI
U+14C1 ᓁ CANADIAN SYLLABICS NAAI
U+14D4 ᓔ CANADIAN SYLLABICS LAAI
U+14EE ᓮ CANADIAN SYLLABICS SAAI
U+1527 ᔧ CANADIAN SYLLABICS YAAI
U+1545 ᕅ CANADIAN SYLLABICS RAAI
U+1554 ᕔ CANADIAN SYLLABICS FAAI
U+157E ᕾ CANADIAN SYLLABICS QAAI
U+158E ᖎ CANADIAN SYLLABICS NGAAI
U+18B0 ᢰ CANADIAN SYLLABICS OY
U+18B1 ᢱ CANADIAN SYLLABICS AY
U+18B2 ᢲ CANADIAN SYLLABICS AAY
U+18B3 ᢳ CANADIAN SYLLABICS WAY
U+18B4 ᢴ CANADIAN SYLLABICS POY
U+18B5 ᢵ CANADIAN SYLLABICS PAY
U+18B6 ᢶ CANADIAN SYLLABICS PWOY
U+18B7 ᢷ CANADIAN SYLLABICS TAY
U+18B8 ᢸ CANADIAN SYLLABICS KAY
U+18B9 ᢹ CANADIAN SYLLABICS KWAY
U+18BA ᢺ CANADIAN SYLLABICS MAY
U+18BB ᢻ CANADIAN SYLLABICS NOY
U+18BC ᢼ CANADIAN SYLLABICS NAY
U+18BD ᢽ CANADIAN SYLLABICS LAY
U+18BE ᢾ CANADIAN SYLLABICS SOY
U+18BF ᢿ CANADIAN SYLLABICS SAY
U+18C0 ᣀ CANADIAN SYLLABICS SHOY
U+18C1 ᣁ CANADIAN SYLLABICS SHAY
U+18C2 ᣂ CANADIAN SYLLABICS SHWOY
U+18C3 ᣃ CANADIAN SYLLABICS YOY
U+18C4 ᣄ CANADIAN SYLLABICS YAY
U+18C5 ᣅ CANADIAN SYLLABICS RAY

Underring
Unicode encodes the underring at U+0325 ◌̥ COMBINING RING BELOW
The underring is used in IPA to indicate voicelessness, and in Indo-European studies or in Sanskrit transliteration (IAST) to indicate syllabicity of r, l, m, n etc. (e.g. r̥ corresponding to IPA [ɹ̩]). R with ring below, L with ring below, R with ring below and macron, and L with ring below and macron were proposed for Unicode because of their use in Sanskrit transliteration and the CSX+ Indic character set. This proposal was rejected, because they are already encoded as sequences.In Pashto romanization, ḁ is used to represent /ə/.Examples:

U+1E00 Ḁ LATIN CAPITAL LETTER A WITH RING BELOW
U+1E01 ḁ LATIN SMALL LETTER A WITH RING BELOW

Emilian-Romagnol
In Romagnol, e̥ is used to represent /ə/ in diphthongs, e.g. Santarcangelo dialect ame̥ig [aˈməiɡ] 'friend', ne̥ud [ˈnəud] 'naked'. In Emilian, e̥ can be used to represent unstressed /ə/ in very accurate transcriptions.

Half rings
Half rings also exist as diacritic marks; these are characters U+0351 ◌͑ COMBINING LEFT HALF RING ABOVE and U+0357 ◌͗ COMBINING RIGHT HALF RING ABOVE. These characters are used in the Uralic Phonetic Alphabet, respectively for mediopalatal pronunciation and strong-onset vowels. These characters may be used in the International Phonetic Alphabet, denoting less and more roundedness, as alternatives to half rings below U+031C ◌̜ COMBINING LEFT HALF RING BELOW and U+0339 ◌̹ COMBINING RIGHT HALF RING BELOW. They are here given with the lowercase a: a͑ and a͗, a̜ and a̹.
U+1E9A ẚ LATIN SMALL LETTER A WITH RIGHT HALF RING is similar in appearance but differs from a͗ because its compatibility decomposition uses U+02BE ʾ MODIFIER LETTER RIGHT HALF RING instead of U+0357 ◌͗ COMBINING RIGHT HALF RING ABOVE.
Other, similar signs are in use in Armenian: the U+0559 ◌ՙ ARMENIAN MODIFIER LETTER LEFT HALF RING and the U+055A ◌՚ ARMENIAN APOSTROPHE.
Breve and inverted breve are also shaped like half rings, respectively, the bottom and top half of a circle.

Other uses
The ring is used in the transliteration of the Abkhaz to represent the letter ҩ. It may also be used in place of the abbreviation symbol ॰ when transliterating the Devanagari alphabet.

Letters with ring
Similar marks
The ring as a diacritic mark should not be confused with the dot or U+0366 ◌ͦ COMBINING LATIN SMALL LETTER O diacritic marks, or with the degree sign °.
The half ring as a diacritic mark should not be confused with the comma or ogonek diacritic marks.

References
External links
Diacritics Project — All you need to design a font with correct accents