(CBS) — As we report on the debate over whether Illinois should apply a penny-per-ounce tax to sweetened beverages, a question arises: what are we talking about?

The pop-versus-soda question has fueled Internet discussions, and is a big part of the work of Allan Metcalf, an English professor at MacMurray College in Jacksonville, Ill.

He is also chief administrative officer of the American Dialect Society.

“It looks like Interstate 55 is a corridor of soda in the northern part of the state with ‘pop’ flanking it on either side,” he says. “How that comes about … I don’t know!”

Metcalf says Chicago says “pop” and St Louis says “soda,” though some St. Louisans say, “sody.”

Metcalf, author of books that include “How We Talk: American Regional English” and “OK: The Improbable Story of America’s Greatest Word,” says the variances seem to fall where drink manufacturers are and whether those makers use pop or soda. In the Deep South, everything’s a Coke, even if it’s a 7-Up.

Graduate student Will Baptist of the University of Illinois at Urbana-Champaign is concerned about this as well. The Baltimore-bred Baptist, an advisor to international students on campus, used to teach English as a Second Language in Japan.

“Pop is a sound and a verb. Soda is what it actually is. Soda is the beverage,” he says.