"a" and "an"

       At a dermatologist's office, I read a brochure on sunscreens that spoke of “a SPF of 15.”  To my speechy mind, that was wrong.  It should have been "an SPF of 15."

       The common thought is that if a word begins with a consonant, the article before it is properly "a".   It's true "s" is a consonant, but that "s" is an initial and is pronounced "ess."

       Before a vowel, the article "a" becomes "an".    I like things in print that recognize the way we’d  say something when reading it aloud (or even when "silent"-reading it for our own inner ear).