In 1981, Mark Venable drove across the U.S.A., a trip that took him from his native Washington state to New England. Somewhere in the Northeast, he crossed an invisible border.

That unseen line snaked south, through Steuben County, N.Y.; Clearfield County, Pa.; Garrett County, Md.; and Grant County, W.Va. On the west side, people employed the same word for a sugary carbonated beverage that Mark had grown up using: “pop.” On the east side of the line, they used a different one: “soda.”

“At least it was all derivative from ‘soda pop,’ ” wrote Mark, who now lives in Arnold, Md. “In New Hampshire I first came across ‘tonic’ as a word for soda pop and not just as that key ingredient of gin and tonic, but for all soft drinks. I saw that in Maine as well. Don’t get me started on calling milkshakes ‘frappes’!”

You can explore the Pop/Soda Line at popvssoda.com, a fun mapping project by cartographer Alan McConchie that includes breakdowns by state and county. Despite the website’s binary name, it includes a third contender, one familiar to Arlington reader Jason Maddux.

“I spent the majority of my youth in Mississippi,” Jason wrote. “There, if you wanted a soft drink at a restaurant, you asked for a ‘coke.’ The server then asked you what kind you wanted, where you specified if you wanted a Coca-Cola or one of the other soft drinks available.”

Jason said he once took a road trip to North Dakota with family friends, stopping somewhere in the Midwest for a bite to eat.

Wrote Jason: “The server asked if I wanted a pop. Horrified, I looked at my friend’s mom thinking the server wanted to hit me. My friend’s mom explained that ‘pop’ was another word for ‘coke.’ Finally, I understood the line from Jimmy Buffett’s ‘Margaritaville’ about stepping ‘on a pop top,’ which had always confused me.”

Another example of travel broadening the mind.

I’ve certainly had my mind broadened by my recent columns on all the different words Americans use. If you really want to get into the weeds, you can explore the six-volume “Dictionary of American Regional English,” published by Harvard University Press.

The publication is based on interviews conducted in more than a thousand communities. The National Museum of Language has a nifty virtual exhibit on how, starting in 1965, DARE fieldworkers spread across the country, mining words from American minds. (Find it under “Exhibits” at languagemuseum.org.)

The DARE project is overseen by the University of Wisconsin at Madison. (That’s in Dane County, a rare Midwestern outpost of pop/soda parity, according to popvssoda.com.) An online subscription to the dictionary is $49 a year. There’s more info at dare.wisc.edu.

David Silber grew up in “pop” territory: Detroit. “A ‘soda’ was an ice cream soda,” he wrote. “More interestingly, a hot dog — always with chili, raw onions and mustard — was a ‘Coney island.’ ”

One year, David was driving through New York state on his way to visit his stepdaughter in Watertown, N.Y. He kept seeing signs for something called “Michiganers,” priced at two for three bucks.

“Finally, curious as to what my native state had contributed to New York cuisine, I stopped and asked,” wrote David, who lives in Bethesda, Md., now. “A ‘Michiganer’ is — yep, you guessed right — a hot dog with chili, mustard and raw onions.”

In a column last week, I quoted Barbara Kagan, who grew up in Levittown, N.Y., calling a playground slide a “sliding pond.” Many readers chimed in with similar memories — and explanations.

David Margulies grew up in Nutley, N.J., and always called the park’s slide a “sliding pond.” Wrote David, who lives in Bethesda now, “My wife, from Buffalo, N.Y., was always baffled, but I later learned this was merely a corruption of ‘slide upon.’ ”

Kee Malesky — a “Brooklynite-in-Exile” now living in the District — said the term comes from the company that made the slides: Slide Upon.

“It didn’t take long for Brooklyn kids — and maybe everywhere in NYC — to turn ‘slide upon’ into ‘sliding pond,’ ” Kee wrote.

In 1983, New York Times writer Jack Rosenthal quoted a DARE editor who suggested that “sliding pond” may have come from Manhattan’s original Dutch settlers. The Dutch word for “slide” is “glijbaan.” Perhaps that morphed into “slide pond.” But were there really slides in 17th-century New Amsterdam?

Wrote Rosenthal: “It’s not hard to see how, as the term was passed down over three centuries, ‘baan’ could [become] ‘pond.’ ”

Perhaps, but that sounds like a stretch to me.

In my admittedly brief search, I couldn’t find a company called Slide Upon. I did find a book from 1909 on playground architecture, complete with diagrams and plans. The authors just used the word “slide.”

I do love the descriptive name for an attraction they called the “slide for life.”

They explained: “The children climb the Incline Ladder, take hold of this ring, and ‘slide for life.’ This piece of apparatus is very popular.”

Today we’d call it a zip line.

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.