Doubling rule (-ff, -ll, -ss, -zz)
Double final f, l, s, and sometimes z immediately following a single vowel in a one-syllable word, as in staff, bluff, tell, still, grass, bliss, buzz, and jazz. (Common exceptions are pal, gal, if, clef, gas, this, us, thus, yes, bus, plus, and quiz. [Although quiz contains two vowel letters, q is always followed by u in English words so only i is considered a vowel in quiz.])