Why is “q” followed by a “u”?

Question

Is there a particular rule that states that q should always be followed by a u? Because in certain cases like Qatar, or qawwali, this so-called rule is violated.

What do you folks say?

Answer 1

There is no rule that q must be followed by u in all circumstances. This is merely true in the vast majority of circumstances, and it goes back to Latin.

The early Latins had three different letters for the [k] sound: C K Q. However, they only had one letter to represent the [u] and [w] (or [v]) sounds: V. It became customary to write the sequence [kw] (which is fairly common in Latin) as QV and all other instances of [k] as C. (K dropped out of use in most words.) This usage survived into most other European languages that were written with the Latin alphabet, though eventually the letter V was differentiated into U and V, and the accepted spelling of [kw] became QU.

Words spelled with Q without U are generally more recent additions to English, and often represent words borrowed from Semitic languages. Those languages are written with non-Latin alphabets and often have more than one [k]-like sound. When transliterating these scripts, K is usually used for [k], and Q for another sound such as [q], a uvular, "guttural k". In romanizations of Chinese Q is also used for a sound similar to the English "ch".

Answer 2

In English, there are no rules about when the q must be followed from a u.

English words use qu when it was present in the original word.

English Word | Original word | Language
quandary     | quando        | Latin
quantify     | quantificare  | Medieval Latin
quantum      | quantus       | Latin
quark        | quark         | German
quarry       | quarreria     | Medieval Latin
quest        | queste        | Old French
question     | question      | Old French
query        | quaerere      | Latin
quetzal      | quetzalli     | Aztec

Other times, English words use qu instead of cw, ku of the original word.

English Word | Original word | Language
quean        | cwene         | Old English
quell        | cwellan       | Old English
queen        | cwēn          | Old English 
quench       | -cwencan      | Old English
quern        | cweorn(e)     | Old English
quiche       | Küchen        | Alsatian dialect
quick        | cwic, cwicu   | Old English