GLOBE CORRESPONDENT

     They form a java-spiked triangle within Davis Square: The Someday, Diesel, and the Elm Street Starbucks.
     ''It's the most caffeinated neighborhood I can think of," laughs Gus Rancatore, owner of the Someday Café.
     The three coffee shops are located within a few blocks of one another, and they are as different from each other as American breakfast blend and double-shot espresso. The one thing they have in common is that they are all packed with customers, loyal to their own, most of whom are hanging out and/or studying and/or keeping up with their cuppa-joe joneses. On Friday nights, finding a good seat at any of them requires the luck and patience of snagging a nearby parking spot.
     There is Starbucks with its hushed corporate color scheme of maroons, communal fireplace, and reliably digestible tunes; the Someday with its dorm-room lounge furniture and anything-goes music playlist; and Diesel with its high ceilings, Day-Glo walls, and gas station steel art, newly expanded by 1,100 square feet.
     Over the droning reverberation of My Morning Jacket, Diesel co-owner Jen Park says she thinks variety helps make Davis Square the local epicenter of coffee consumption.
     ''The level of competition that's out there really makes it better for us, because I think if we were the only coffee shop in the area, it would be a really different kind of environment," she says. The Someday is down the street and Starbucks moved in across the street six months after Diesel opened.
     Someday's Rancatore says there are plenty of customers to go around and the competition is friendly. Employees often take their breaks at the ''competing" coffee shops. As he talks about his competitors, he is suddenly reminded that he has a borrowed ladder to return to the Diesel folks.
     Sarah Otto, 24, of Cambridge, who is in her second year of a master's program at Harvard Divinity School, estimates she spends ''at least four to five days a week" in Diesel.
     ''I have pretty much written all of my papers since I started my graduate program here," she says. ''The atmosphere has always just worked well for me. I can't work in a library where it's deathly quiet."
     Kate Bovitch, 22, of the Fenway, is a series of contradictions. She's not a coffee drinker, but is a Starbucks worker and Diesel devotee. A half-eaten sandwich and a cup of melting ice cubes with a sip of soda sit in front of her as she talks about how Diesel's atmosphere has worked well for her creative-writing endeavors.
     ''I find it conducive to what I'm doing because there's enough white noise not to have to pay attention to any of it," she says.
     But for Meg Lovejoy, 37, of Somerville, Diesel's noise is too much. Sitting in the quiet of Starbucks across the street, working on her dissertation, she might as well be a world away.
     ''Diesel Café doesn't seem very comfortable to me. The acoustics aren't very good, and I feel like I can hear all the conversations around me. This just feels more relaxing," she says, patting the armrests of her puffy chair.
     Diesel's décor is part barroom and part carnival, with red velvet pool tables, a retro photo booth, and vibrant art of subjects ranging from Weeble toys to Cracker Jack prizes.
     Lovejoy says she finds Starbucks' big, comfortable chairs appealing, but she doesn't like ''supporting the mega-corporation system."
     Laurinda Bedingfield, 51, also of Somerville, has no shame about supporting Starbucks. Probably because they support her, she says. The Davis Square Starbucks displays several works of art by her and other local artists.
     Bedingfield says she likes Diesel, but ''it's not that comfy," and she likes Someday's coffee, but ''the scene is kind of grungy in there." Someday is partially defined by its overflowing bulletin boards and rotating art displays. This month the café features photos by Marta Fodor.
     All of the cafés encourage customers to linger. Park says one of the reasons Diesel serves food is so customers can have breakfast, lunch, and dinner there. ''You can really hang out here for a full day," she says.
     Defining the diverse groups who make up the coffee shop clientele can be a daunting task. Rancatore gives it a shot.
     ''Someday [Café] in the morning is people getting their coffee and going off to their jobs, but at the other extreme are the people hanging out in front of the store who tend to look like skateboarders, street musicians, and punks. But while they're in front of the store, if you come into the Someday at night, it's surprising that there are a lot of conventional students studying in study groups," he says.
     ''Diesel, which at certain points has been described as a feminist-lesbian coffee shop, is really only that for part of the night," he continues. ''If you go there on Friday and Saturday nights, it's a bit of a younger crowd and it skews more towards women but in the morning and the day, it's a different group."
     Jody Cornish, 32, a consultant and part-time grad student working on her laptop at Starbucks, says the different shops serve different purposes. ''This place is for working, but I go to Someday to hang out with friends."
     Park agrees that the only consistency is variety. ''It's pretty amazing the range of people we get," she says. ''There are lots of young moms who come here with their kids, and lots of young professionals. I would really have a hard time classifying."
     Brian Morris, a daily Someday customer in his 50s, speaks above the Ziggy Marley CD vibrating through the café and says the atmosphere there is ''kind of like a melting pot."
     The customers are the cafés' best advertisers, says Rancatore.
     ''You see people in there and you might be in a rush, but you think to yourself, 'maybe I'll go in there when I'm not in such a hurry.' "
     Just outside the triangle, a couple of other shops also serve mean cups of bean. The chemical-conscientious O'Natural, which opened earlier this month at the site of the former Carberry's Bakery & Coffee further down Elm Street, offers a place to linger with a book or laptop -- though just until 8 p.m. most nights. And right next to the Davis Square T stop is Au Bon Pain, with an early (7 p.m.) closing time but outdoor seating that suddenly came back in style last week