EDGARTOWN, Mass. — They’re a natural extension of life if you grew up in Boston or wherever Red Sox caps are ubiquitous, and if Cape Cod is too bourgeois.

Nantucket and Martha’s Vineyard. The very names, for many of us from other places, conjure thoughts of lobsters and clams and harpooners and presidential getaways and weathered houses facing the salty sea and solitary walks on beaches and — at least with Nantucket — limericks.

A gentleman came to Nantucket

With nil but a shovel and bucket ...

Etc.

But, aside from both being gorgeous and islands we’d all love to spend summers on if we could, what are these places, really? How is Nantucket different from Martha’s Vineyard? Is there actually a vineyard on Martha’s Vineyard? (Hint: There isn’t.) Can you afford a weathered house facing the salty sea on Nantucket? (Hint: Got a few million in the trust fund?)

The basics: They’re both islands south of Cape Cod — not too far, but Nantucket more distant than Martha’s Vineyard — and reachable by ferries and other watercraft or, if you’re in a hurry or don’t like boats and do like little airplanes, by air.

Nantucket? An island with one town — simply “Town” to locals — full of perfectly groomed houses pleasant to look at (which is why the whole island is a National Historic Landmark District), fresh seafood and enough places to spend a night or three or a season. The island is basically flat, aside from bluffs overlooking the sea, a sea that’s menacing some of those bluffs and forcing homeowners to move their houses back a bit, lest they tumble into worthlessness. Great bike trails. Good beaches.

Martha’s Vineyard? An island about twice the size of Nantucket, with three towns — Edgartown, Vineyard Haven and Oak Bluffs — plus three additional villages, more cool houses, fresh seafood and enough places to spend a night or three or a season. Unlike on Nantucket, there are hilly areas and winding roads bordered by low stone walls, and there are small farms. The only mini-golf course (and it’s really mini) on either island is here. Great bike trails. Good beaches.

And Martha’s Vineyard co-starred with Roy Scheider and a fake shark in “Jaws.” The association is a tired cliche — the movie is 42 years old, people — but shops there still peddle “Amity Island” signs, so ...

“This bridge coming up is affectionately known as the ‘Jaws Bridge,’?” Nancy Gardella, of the island’s chamber of commerce, said during a drive-around. If you’ve seen the movie, you know the bridge. “And despite the fact that there’s a ‘Keep off the bridge, no jumping or diving’ sign, typically in summer, the bridge is lined with kids who stand along the edge and jump off.”

Which is a reminder: Both islands are fine for kids.

And both share a nautical, grassy-duned, artsy-literary and just-painted sensibility. Each summer, the population of these islands routinely jumps from a combined 25,000 or so to five or six times that, and it’s not just the beaches.

What’s kind of curious is that despite their proximity, about 70 minutes apart by high-speed ferry, islanders tend to stick to their own turf. Dick Burns is a bookseller at Mitchell’s Book Corner in Nantucket town.

“We’ve got Nantucket,” he explained. “I’ve spent, literally, about six hours on the Vineyard.” He’s lived on Nantucket since 1972.

Betsy Larsen works the family’s namesake fish market in Menemsha, a dot on Martha’s Vineyard alongside one of the island’s best sunset beaches. She gets across to Nantucket but only occasionally. She prefers the Vineyard.

“It’s just unique,” Larsen said as she ladled her shop’s clam chowder into a Styrofoam bowl. “I love Nantucket, but Nantucket’s a totally different vibe.”

The closest thing to Nantucket town on the Vineyard is Edgartown. Like Nantucket, it’s a village of Cape Cod-ish and colonial houses that beg to be photographed. As with its cousin, it has restaurants (some high-end), shopping and galleries. The Old Whaling Church (1843) is a whale of a church that doubles as a concert venue and makes a nice counterpoint to Nantucket’s newer Whaling Museum; smaller St. Andrews Episcopal Church (1899) boasts two stained-glass windows autographed by Louis Tiffany.

A 15-minute walk from the center, on its own beach, is Edgartown Lighthouse, one of five on the island (Nantucket has three), and on this day, Walt Looney was releasing a slightly undersized striped bass he’d just coaxed to the sand.

“I was hoping to catch a bluefish for dinner,” said Looney, a lawyer who’s been coming to the Vineyard for 25 years. “I live here now. I haven’t been to Nantucket in a long, long time.”

As for the island’s two other towns and three smaller ones, each is a little different from the others, which makes them worth poking around.

There’s no denying Nantucket’s cobblestone charm. There’s also no denying Nantucket’s corporate pedigree. Writers (William Styron, Lillian Hellman, more) and celebrities (Jackie Kennedy Onassis, John Belushi, more) are or were drawn to the Vineyard and its famously tolerant villages, but the serious money (Jack Welch, the Heinzes, more) has tended toward Nantucket and its $10 million shanties.

What both have is bargain-priced bus service that makes cars optional and is shared by everyone — among them, one day on Martha’s Vineyard, a gentleman named Michael Haydn, who boarded the No. 1 between Edgartown and the ferry port at Vineyard Haven and slipped seamlessly into conversation.

“The red roof through the trees there? John and Barbara Hersey. Friends of my parents’,” he began. For a time he’d been a taxi driver. “I drove Mike Wallace. And I drove the late Beverly Sills. Her friend from San Miguel de Allende was my mother’s college roommate.”

I asked, inevitably, about the impact on the island of “Jaws.”

“I’m in there,” he said. “Guitarist, in the opening scene.”

Yep, he’s in there: 14 seconds on guitar, near the bonfire, billed as Mike Haydn, his hair not yet gray ...

There was a guitarist named Haydn

Playing tunes for a shark-destined maiden ...

Etc.

Oh: The fishing is excellent off both islands. And if you insist, on either island you can charter a bigger boat.

Alan Solomon is a freelance writer.