In Philadelphia's thriving craft-beer scene, where hundreds of bars with formidable tap lists vie for the dollars of discerning drinkers, one of the toughest tasks has become to find a way to stand out.

Though competition tends to breed excellence, there's also a by-product: collaboration.

Increasingly, Philadelphia bars, stalwarts and upstarts alike, are teaming with brewers to create proprietary products - special beers exclusive to specific venues, often made to the nitty-gritty specifications of the people buying them. The trend is about more than just exclusivity: There are business, interpersonal, and creative benefits to this distinct symbiosis, links in a chain that illustrate the premium placed on relationships within the beer world.

"For local bars, it's so hard to have anything that's unique anymore," says Jared Littman, who, with wife Kristy, runs, a real-time draft-list resource for drinkers in the region. "Because there are so many craft-beer bars in Philadelphia, to have something that other bars don't is close to impossible."

Bar owners working directly with breweries is not new. Monk's Cafe got in on it early, around 2000, when co-owner Tom Peters was visiting Belgium and tried a traditional sour called Bios, produced by Brouwerij Van Steenberge, outside Bruges. "They were going to stop making it because there was no market for it [in Belgium]," says Peters, who struck a deal to bring the product to America under the name Monk's Cafe Flemish Sour Ale. It's become a cornerstone of the Monk's brand, always on tap at the bar, but also sold around the world in bottles.

Peters also has teamed with breweries like Allagash, Cantillon, Russian River, and Dogfish Head to produce original limited-edition beers, often poured at special events. Though he likes to get hands-on with the process, "You have to rely upon the brewer to articulate what your end goal is," Peters says.

There are other examples of long-standing "house beers" provided exclusively to one client. Manayunk Brewing Co. produces the 4.5 percent ABV American Sardine Ale, a tweak of its standard session ale, for American Sardine Bar in Point Breeze. Stoudts of Adamstown sells its seasonal Oktoberfest year-round to Fairmount's London Grill, where it's listed as Willie Sutton Lager. "It's a standard for regulars," London co-owner Terry Berch McNally says of the easy-drinking 5 percent ABV beer.

With more recent instances of this practice, however, the level of customization afforded bar owners has increased.

"The day in, day out gets a little predictable, so doing something new is always fun," says Brendan Hartranft, who makes collaborative beers a prominent part of the draft lineup at Memphis Taproom, Local 44, Coeur, Strangelove's, and Clarkville. During the coming Philly Beer Week, he'll roll out a number of one-offs, such as Eden, a saison with English hops and white pepper he dreamed up with John Stemler of Perkasie's Free Will. Round Robin Lager, an ongoing project with Victory Brewing Co., releases renditions based on the personal preferences of Hartranft, Standard Tap owner William Reed, and Vetri Family beverage manager Steve Wildy.

Brewers and bar owners "like collaborating on these. . . . They like to have something that's stretching them a little bit," Hartranft says. "And we always try to play to a brewery's strengths."

On the production side of things, the practice makes considerable dollars-and-cents sense. "It's a symbol of loyalty and commitment to each other, [but] it has business value, too," says PhillyTapFinder's Littman. "Breweries like it because they have essentially scored a consistent [draft] line."

Wildy, of Vetri, first experienced the appeal of an exclusive during the 2012 opening of North Broad Street beer bar Alla Spina. Basing the recipe on Marc Vetri's personal specifications, Victory produced Novello, a "yeasty but drinkable" ale similar to the flagship Golden Monkey.

"It took off more than we anticipated," says Wildy, who noticed that those ordering Novello tended to stick with it through an entire drinking session. "People would drink it and say, 'This is cool.' Five beers later, they'd say, 'This is really cool.' That became the touchstone for what we wanted these collaborative beers to be." Currently, Alla Spina is pouring Victory's Verde, a tart saison also made exclusively for it.

Yards, which also has provided custom beers for Alla Spina, is perhaps the most prolific local contributor to the collaborative category. It is large enough to have a separate three-barrel system dedicated solely to special projects. The brewery has cranked out custom beers for dozens of bars, and it isn't afraid to take things left field. "We use it as joint experimentation," sales manager Rick Anstotz says.

Yards also brews exclusives with a mind toward the season. Independence Beer Garden this year will pour a grapefruit pale ale, which brewer Tim Roberts flavored with a puree of the fruit, plus the zest and three varieties of hops. Spruce Street Harbor Park, meanwhile, has its hands on a beer brewed with blood orange. Aside from bars, Yards also has produced special beers for the Sons of Ben, the Philadelphia Union soccer team fan club, and the metal band In Flames.

To celebrate the 10th anniversary of the bar Good Dog, Yards crafted a nitro pale ale and an imperial stout - the preferred styles of owners Heather Gleason and Dave Garry, respectively. They could be drunk separately or combined into one glass as a black-and-tan.

In an industry dominated by personal connections, collaborative brews are an intellectually stimulating means to a mutually beneficial end. "If you're involved in something," Hartranft says, "you're more likely to be excited about it."