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Ch is a digraph in the Latin script. It is treated as a letter of its own in Chamorro, Czech, Slovak, Igbo, Quechua, Guarani, Welsh, Cornish, Breton and Belarusian Łacinka alphabets. In Vietnamese and Spanish, it also used to be considered a letter for collation purposes but this is no longer common.
Voiceless velar fricative 
In the Goidelic languages, several Germanic languages, many Slavic languages that use the Latin alphabet instead of the Cyrillic alphabet, Welsh, and others, ch represents the voiceless velar fricative [x]. Additionally, "ch" is frequently used in transliterating into many European languages from Greek, Hebrew, Yiddish, Arabic, and various others.
In Rheinische Dokumenta, ch represents [x], as opposed to ch, which stands for [ç].
In English words of French origin, "ch" represents [ʃ], as in charade or machine. This pronunciation occurs in just a few loan words from other sources, like machete (from Spanish) and pistachio (from Italian).
This digraph should not be confused with c'h [x].
Ch is the fourth letter of the Chamorro language and its sound is [ts].
The letter ch is a digraph consisting of the sequence of Latin alphabet graphemes C and H, however it is a single phoneme (pronounced as a voiceless velar fricative [x]) and represents a single entity in Czech collation order, inserted between H and I. In capitalized form, Ch is used at the beginning of a sentence (Chechtal se. "He giggled."), while CH or Ch can be used for standalone letter in lists etc. and only fully capitalized CH is used when the letter is a part of an abbreviation (e.g. CHKO Beskydy).
The letter Ch is equal to other letters of the Czech alphabet. It comes between H and I. Thus, the word chemie "chemistry" comes after fyzika "physics" in an alphabetical list. Names beginning with Ch are listed in the same way in a phonebook. In a crossword it takes only one square. Only few Czech words treat CH as two separate letters, e.g., puchoblík, from pucovat (German putzen "clean") and hoblík "plane".
In the 15th century, the Czech language used to contain many digraphs like modern Polish does but most of them were replaced by single letters with diacritic marks by the reform of Jan Hus. Besides ch, there is only one digraph used in the Czech language - dž, representing voiced postalveolar affricate. However, ch is the only Czech digraph which is treated as a single letter while dž is used in translating a foreign word into Czech (to approximate a foreign phonetic sound that has no Czech counterpart e.g. jam in Czech is džem).
Dutch ch was originally voiceless, while g was voiced. In the northern Netherlands (and standard Netherlands Dutch), both ch and g are voiceless, while in the southern Netherlands and Flanders the voiceless/voiced distinction is upheld. The voiceless fricative is pronounced [x] or [χ] in the north and [ç] in the south, while the voiced fricative is pronounced [ɣ] in the north (i.e. the northern parts of the area that still has this distinction) and [ʝ] in the south. This difference of pronunciation is called 'hard and soft g'.
In native French words, ch represents [ʃ] as in chanson (song).
In words of Greek origin, it represents [k] as in archéologie.
In German, ch represents two allophones: the voiceless velar fricative [x] when following back vowels or [a] (the so-called "Ach-Laut") and the voiceless palatal fricative [ç] in all other positions (the so-called "Ich-Laut"). A similar allophonic variation is assumed to have existed in Old English.
In German, it represents [k] before -s, as in Fuchs (fox). An initial Ch (which only appears in loanwords) may also be pronounced [k] in southern varieties, and is always pronounced [k] when a consonant follows the initial Ch as in Christus or Chlor (chlorine).
The Rheinische Dokumenta writing system uses ch, for the voiceless palatal fricative [ç], while ch represents [x].
In Interlingua, ch before e and i represents the sound [k].
The Romans used ch to transliterate the sound of the Greek letter chi in words borrowed from that language. In classical times, Greeks pronounced this as an aspirated voiceless velar plosive [kʰ]. In post-classical Greek (Koine and Modern) this sound developed into a fricative.
Nguni languages 
Ch has been used in the Polish language to represent the "soft h" /x/ as it is pronounced in the Polish word chleb "bread", and the h to represent "hard h", /ɦ/ where it is distinct, as it is pronounced in the Polish word hak "hook". Between World War I and World War II, the Polish intelligentsia used to exaggerate the "hardness" of the hard Polish h to aid themselves in proper spelling. In most present-day Polish dialects, however, ch and h are uniformly collapsed as /x/.
In Portuguese, ch represents [ʃ].
Separate letter 
ch has its own name (che) and used to be treated as a distinct letter of the alphabet. While Ch is used at the beginning of a sentence, either Ch or CH may be used for a standalone letter in lists, etc. In a normal Spanish crossword, 'CH' takes up two squares, although in some old crosswords it occupied only one square.
Until 1994 ch was also treated as a single letter in Spanish collation order, inserted between C and D; in this way, mancha was after manco and before manda. There was similar special treatment for ll. However, an April 1994 vote in the 10th Congress of the Association of Spanish Language Academies adopted the standard international collation rules, so ch is now considered a sequence of two distinct characters, and dictionaries now place words starting with ch- between those starting with cg- and ci-. Similarly, mancha now precedes manco in alphabetical order.
In Slovak, ch represents /x/, and more specifically [ɣ] in voiced position. At the beginning of a sentence it is used in two different variants: CH or Ch. It can be followed by a consonant (chrbtica "spine"), a vowel (chémia "chemistry") or diphthong (chiazmus "chiasmus").
Only few Slovak words treat CH as two separate letters, e.g., viachlasný (e.g. "multivocal" performance), from viac ("multi") and hlas ("voice").
In the Slovak alphabet, it comes between H and I.
Upper Sorbian 
"Ch" represents [kʰ] in Upper Sorbian.
Alternate representations 
International Morse code provides a unitary code for Ch used in several non-English languages, namely — — — —.
In the Czech extension to Braille the letter Ch is represented as the dot pattern ⠻. English literary braille also has a single cell dedicated to <ch> (dots 1–6), which stands for "child" in isolation, but this is considered a single-cell contraction rather than a separate letter.
Pop culture 
All principal characters created by Roberto Gómez Bolaños for his TV shows have names starting with Ch, including Chómpiras, Dr. Chapatín, and perhaps most famously El Chavo and El Chapulín Colorado, a superhero whose costume has a "CH" inscribed by a heart (analogous to the way Superman's costume has an S inscribed on a diamond). Bolaños' artistic name was Chespirito, also with a Ch (Chespir would be a Spanish substandard pronunciation of Shakespeare; suffix -ito means "little").
See also 
- Association of Spanish Language Academies, official website