The best ideas percolate when Jules Markey walks his Center City postal route. Down and across zip code 19130, he fills the silvery stacked mailboxes of five high-rise apartment buildings on Spring Garden Street. Puzzles help Markey combat the repetition.

The other day, as he slipped an Entertainment Weekly into a mail slot on Brandywine Street, he remembered when trash served as inspiration for the first crossword of his the New York Times accepted.

A resident on his route had discarded a pile of doors by the curb.

"So I came up with a bunch of words that precede the word door," Markey recalled. He scribbled them on postal labels stuffed in his shirt pocket.

Barn door and storm door made "barnstorm." Dutch door and oven door made "dutch oven." The hint at 61 Down - "Welcoming symbol . . . or what each part of the six answers to the starred clues can do?" - indicated the theme:

"Open door."

And so the 58-year-old mailman's first professional crossword was born. (He was paid $300.)

"It is surprising, isn't it?" said Jerome Jacobstein, a retired doctor whose mail Markey carries. "But he's really good at it. If he's making difficult puzzles, which he is, then that means he has a good vocabulary and a good range of knowledge."

Not just really good.

"These," said Will Shortz, the Times' venerable crossword puzzle editor, "are expert constructions."

Markey - so lanky he reaches over three steps to slide the mail through door slots - sees the puzzles as both challenge and affirmation.

"You complete the puzzle and you say, 'Hey, pretty good.' I don't get that at my job," he said. "I think that's part of it. I always liked doing it because it validated that I did know a lot. Some people have that in their job; some people don't."

Markey has never met Brenda Gray. She was just another name on his route - until the day last spring when Markey sorted her mail and noticed she subscribed to Collector's Crosswords.

Another aficionado.

"Would you like to do my puzzles?" he asked.

She loved the idea.

"I just thought it was a very charming thing he did," said Gray, a retired librarian. He left her copies of his crosswords published in the New York Times.

There was his debut puzzle with 27 Across: eight letters for "Facilities housing large planes?" And the rebus puzzle that, once solved, revealed "Post Office boxes" across the center. The one with five answers that related to a class of mail service. The theme, USPS, fit at 5 Across.

She returned the crosswords completed, with comments.

Markey's route has not changed much in 23 years. Other customers discovered his hobby; he revels in explaining the process. Small talk with the apartment-building concierges gravitates to puzzles.

"Especially in a big city like this," Gray said, "you wouldn't expect that kind of personal touch."

The "door" puzzle ran Feb. 14, 2013. In addition to the eight the Times has published, he's had four more accepted that are yet to be printed.

Markey's wife, Piljo, threw him a party at their Blue Bell home after the first puzzle was published May 10, 2012. Only a few friends, including a doctor, were able to solve it. Markey printed it out for his colleagues at the Fairmount post office on 19th Street.

"And no one could solve it," he said. "It was too damn hard. It really was. It was a hard puzzle."

Job security is what led Markey to the U.S. Postal Service at 34. Born in Brooklyn, he attended the University at Albany and earned a mathematics degree. He joined a cousin who started a property-management business in Philadelphia. That fizzled. Then, he painted houses.

His post office orientation, 25 years ago, included speeches from a series of union officials.

One message in particular stuck, he recalled:

" 'Some people do work that satisfies them. They get up in the morning, they love going to work, and it's fulfilling. Other people, they have things in their life that are satisfying and fulfilling. Their jobs help them to do it.' "

When Markey served as a judge at last year's American Crossword Puzzle Tournament, he discovered that crossword constructors come from all backgrounds. He met a furniture-store owner from Rochester, N.Y., a chemistry professor at the University of Minnesota, a corporate brander from San Francisco.

Those are indoor jobs. Markey likes his independence, being outside, seeing things that spark his next puzzler.

"He definitely has a playful mind," Shortz said.

The hardest part, Markey says, is finding the right theme. Barbara Keyser, another friend on the route, has pitched a few for him to use in puzzles. "So far," she said, "he has rejected everything."

There are websites Markey will consult for help, but he will not use crossword-compiler software to construct his grids.

"I like the type of clue that sends you in one direction," Markey said. "A misdirection type of clue."

Like the answer to 27 Across from his debut puzzle, the type of deception that makes Markey - and his crossword disciples - chuckle. Facilities housing large planes?