Why is the plural of "quiz" spelled with double Z?

The plural of "quiz" is spelled with double "z" while the plural of "box" (and sometimes "bus") is spelled with a single last consonant. Why is it so? Is this the general rule to double the last consonant to keep the syllable closed?

Solution 1:

In most cases where a word ends in (vowel)-(consonant)-e, we pronounce the last syllable with a long vowel sound. Conversely, most words that end with a double consonant get a short vowel sound.

So: when adding "es", "er", "est", or "ed" to the end of the word would appear to change the vowel sound, double the consonant.

Examples:

• quizes - ize is usually pronounced like "eyes", so change it to quizzes to preserve the short I sound

• subed (short for "substituted") - ube is usually pronounced "oob" or "yoob", so change it to subbed

• biger - ige is usually pronounced "eyej", so change to bigger

Related: Tom Lehrer's song Silent E from The Electric Company TV show (one of my childhood faves!)