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