Why does the plural form of quiz have a double z?

It follows the standard rule: monosyllabic Words ending with a single 's' or 'z' double that consonant and add 'es' to form the plural. This is the logic: if a Word ends in an 's' or 'z' you cannot just add another 's' to form the plural - it would be unpronounceable. Therefore you need to add 'es'. However, if you add 'es' to a monosyllable ending in a single consonant, the 'e' will turn the vowel into a long vowel. Therefore you must double the final 's' or 'z' to keep the vowel sound short when you add the 'es'. If you did not double the 'z' of 'quiz' when you formed the plural, it would be pronounced 'kwy-ziz' instead of 'kwi-ziz'.