What's the deal with Q?

Question

In all forms of Latin I know, the letter Q is always followed by a U. No other letter seems to be bound this way. The combination QU stands for something like /kw/, and it would make more sense to me to let the letter Q alone stand for /kw/ and absorb the letter U into it since they always come together. This would not be the only letter to stand for two sounds; consider X. The only point in the letter Q seems to be that it signifies that the following U is consonantal, as opposed to C whose following U seems to be always vocalic. For example, consider the difference between qui and cui. This makes Q look like a special case of C, but weirdly so, as it only modifies the pronunciation of a following U, not the C itself.

What is the origin of the letter Q? Why does it behave in such a weird way, always requiring a U? Why do we need the letter (in its current form) in the first place? Can the origins of Latin pronunciation or alphabet shed some light on the unusual behavior of Q? In short: What's the deal with Q?

Answer 1

This answer already gave the answer as pointed by sumelic.

In short, in archaic Latin, there were three letters 𐌂, 𐌊, and 𐌒, corresponding to C, K and Q. All those letters were used for both the sound /k/ and /g/. Later the letter G was used for the sound /g/ so C was the only letter for /k/. The letter K became rare and remained only before /a/ (Kalenda…) and Q, which was used in Etruscan before rounded vowels /kʷ/, remained only before /u/ and was therefore only conserved in QV.

Cf. also Wikipedia:

The Etruscans used Q in conjunction with V to represent /kʷ/, and this usage was copied by the Romans with the rest of their alphabet. In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the two sounds /k/ and /ɡ/, which were not differentiated in writing. Of these, Q was used before a rounded vowel (e.g. ⟨EQO⟩ 'ego'), K before /a/, and C elsewhere. Later, the use of C (and its variant G) replaced most usages of K and Q: Q survived only to represent /k/ when immediately followed by a /w/ sound.

Answer 2

The Latin alphabet is based on the Etruscan alphabet, which had three signs for the /k/ sound: C (𐌂), K (𐌊), and Q (𐌒). The reason for that is that the Western Greek alphabet the Etruscan alphabet was based on used C for /g/ (i.e. Γ), which is a sound Etruscan did not have, and already had two signs for /k/, K (Κ) and Q (Ϙ). The reason the Western Greek alphabet had both K and Q is because in the Phoenician alphabet it was based on these signs (kaph 𐤊 and qoph 𐤒) represented separate sounds, with kaph being a regular /k/ and qoph originally being the emphatic equivalent, which later became the voiceless uvular plosive /q/.

	Phoenician	Greek	Etruscan	Latin
C	𐤂 /g/	Γ /g/	𐌂 /k/	C /k/, /g/
K	𐤊 /k/	Κ /k/	𐌊 /k/	K /k/
Q	𐤒 /q/	Ϙ /k/	𐌒 /k/	Q /k/ (but see below)

None of these distinctions were phonemic in Etruscan (Latin did have /g/, so it later needed to innovate the G sign by adding a horizontal line to C; initially it used C for both /k/ and /g/), but phonetically /k/ was perhaps realised as [q] before rounded vowels (just o in Etruscan; in Latin also u) and consonantal u, so it used Q in those positions, and early Latin inscriptions mostly copied that. K was used before /a/, and C elsewhere.
Later, spelling fashions changed as people realised they really didn't need three signs for one phoneme and C began to be used pretty much everywhere except before consonantal u, where Q was kept. The reason Q was kept there is because the velar in qu actually isn't the phoneme /k/ in Latin; actually, rather than the consonant cluster /kw/, qu represents a single phoneme /kʷ/. We know this from e.g. poetic scansion, where a short vowel followed by qu doesn't become heavy by position as it would if it were followed by two consonants.

It probably would have made more sense to start writing /kʷ/ as just q instead of qu, /g/ as C, and use K for everything else (Arma virumqe kano, Troiae qi primus ab oris &c.), but historical baggage is hard to shed.

(Latin using digraphs to represent sounds that are awkward fits in the Etruscan alphabet has precedent: the Praeneste fibula famously writes Old Latin fefaked as FHEFHAKED, and this is because Etruscan 𐌅 actually represents /w/, not /f/; Etruscan doesn't have /f/.)

Answer 3

From a synchronic point of view, the retention of Q as an allograph of C had the advantage that it disambiguates the distinction of the very common words QVI (qui) and CVI (cui), which would otherwise be homographs.

Answer 4

Q means that the U following it is a consonant.* A U preceded by a C is a vowel.

So, qui is pronounced /kwi/ while cui is pronounced /kuj/.

*_{But see Cairnarvon's more detailed answer. Really the U after a Q is less than a consonant.}