system
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human
ELI5: Why does the letter 'Q' always needs to be followed my the letter 'U' for every word in the English language?

Never understood this rule. Its the only letter that needs to be paired together. I cant think of any words that are just Q without the U. Why are these two inseparable!! I need to know why!!
gpt
This is a leftover from Latin's idiosyncratic spelling rules, which were themselves in part due to history even further back.

The Phoenician script had two separate letters kāp and qōp for the two separate sounds /k/ and /q/. When the Greeks borrowed this script to write Greek, they only had /k/, but for a while they kept qoppa, the Greek version of qōp, around to write /k/ before back vowels (/u o/) and used kappa (the Greek version of kāp) for /k/ before other vowels. After a while, they dropped qoppa and used kappa for /k/ everywhere.

Before qoppa was dropped, though, the Etruscans borrowed Greek letters to write Etruscan, and brought along both kappa and qoppa. Additionally, Etruscan had no /g/ and thus used Greek gamma (used in Greek for /g/) to write /k/ as well. When the Romans borrowed Etruscan letters, they had three different letters for /k/ - modern <c k q> - and used all of them for both /k/ and /g/, such that <c> was before front vowels /i e/, <q> was before back vowels /u o/, and <k> was before /a/.

After a while, though, the system was altered such that <c> became used for /k/ almost everywhere (and later the new letter <g> was made for /g/). The old letters <k> and <q> were thus deprecated, except in two circumstances: <k> hung around in a few fossilised words like *kalendae* 'the first day of the month', and <q> hung around before /u/ when /u/ was followed by another vowel (and thus pronounced like \[w\]). This <qu> spelling for /kw/ has been preserved in words borrowed straight from Latin ever since, and entered native English words through the French-based respelling English experienced during the transition from Old to Middle English after the Norman conquest. Thus, <q> remains in use, but the only environment where it appears is before <u>.

Note that there are words that are present in English dictionaries where <q> is *not* followed by <u>, but these are all loanwords from other languages where <q> has some other value (e.g. *qi*, where Pīnyīn <q> has the value /tɕʰ/!), and most of these loans aren't really nativised in English.

<qu> is used in a number of Romance languages to write /k/ in some environments because Latin /k/ changed to one of several different sounds before /i e/, and then later /kw/ became /k/ in that same environment. Since the spelling of <c> for historical \*k wasn't replaced even when the sound was no longer /k/, this meant that <qu> was the only reliable way to write /k/ before /i e/.

TL;DR: Latin had too many ways to spell the sound /k/ because of the various languages that had used these letters before they got to Latin, and it ended up using <q> in only one specific situation.