50/50

It’s around 6 p.m. on a frigid recent Saturday evening, and the Alchemist Restaurant and Lounge (435 South Huntington Ave., Jamaica Plain. 617-477-6741. alchemistlounge.com) is busy with diners. Families, couples, groups of friends — all ages, from teens to Baby Boomers — tuck in as Elvis Costello’s bouncy ‘‘Oliver’s Army’’ trundles discreetly from the sound system.

As the hours go by, the click of cutlery on china is replaced more and more by the clink of ice on glass as dinner turns to drinks. As with its food, the bar offers lovely twists on classics. The potent and tasty Kicking Mule cocktail turns out to be a fabulously warming concoction that fuses vodka with ginger beer and fresh ginger.

By 11 p.m., with the plates cleared, DJ Christine is commanding the action in the bar area and is spinning tunes by the Ramones, the Clash, Madonna, and more. The Alchemist’s atmosphere has revved up several notches, and while some people relax and recline in the plush booths, others dance and cavort in the space near the windows.

This kind of entertainment alchemy, where a convivial restaurant becomes a lively club, is known as 50/50. It’s the higher-energy, modern slant on the traditional American supper club, really. But this version attempts to attract two different crowds, one that’s here for a good meal and then, later, another that wants to dance, mingle, and live it up a little. Somehow, to keep everyone happy and coming back for more, the two have to be spliced seamlessly.

This 50/50 concept, from a business angle, eradicates those post-dining down times, fills up pre-club gaps and has become the MO for many mid-priced restaurants. After 20 years as a successful Back Bay neighborhood restaurant, 29 Newbury (29 Newbury St., Boston. 617-536-0290. 29newbury.com) is set to close in early February for a major refit and rethink.

Just before Christmas, owner Debbie Lewis introduced Lounge Night, every Thursday, to test the waters on a move outside of the standard bar/restaurant mode. After 10 p.m., tables in the dining room are moved aside to create more mingling room. The music, relayed by an iPod that’s specially programmed by New York DJ/producer Todd Buchler to create a jazzy, funky ambience, moves from background to a focus. Not too much of a focus, though. The aim is for congenial conversation rather than to launch downright dance-floor boogie. ‘‘I’m not out to attract a wild club crowd,’’ says Lewis with a laugh.

Lounge Night was such a hit that it’ll be a permanent fixture, possibly expanded to other nights, too, after 29’s refurbishment with an elegant new marbled interior and fresh neutral paintwork. The redesign will give more flow through the room, which now has a formal, segregated dining area and bar. Specially designed mobile furniture will aid the transition from dining to a late-evening lounge atmosphere, too.

‘‘It’s a very different crowd, the late-night crowd. It’s a socialization time,’’ Lewis adds. ‘‘I want to create the perfect atmosphere for that.’’

As exciting as it is, Lewis is very careful not to upset the comfort zone of her existing dining customers.

‘‘I know my niche,’’ she says. ‘‘Weekends, diners tend to eat later and stay longer. I don’t want to be thought of totally as a lounge, so I won’t make the lounge side a constant. I want it to be special and people will think, ‘Oh, it’s Thursday, let’s go to 29.’’’

The half-and-half approach has already caught on with some restaurants. Sonsie (327 Newbury St., Boston. 617-351-2500. sonsieboston.com), 33 Restaurant and Lounge (33 Stanhope St., Boston. 617-572-3311. 33restaurant.com), and now OM (92 Winthrop St., Cambridge. 617-576-2800. omrestaurant.com) are handling dining and nightlife audiences. OM recently added DJs to spin eclectic house in its downstairs lounge on Friday and Saturday nights, from 11 until closing.

‘‘It’s going extremely well, especially as we just started a couple of weeks ago and most of the promotion has been by word of mouth,’’ says Bik Yonjan, OM’s owner.

So far, diners have responded positively.

‘‘We are very careful to not have the music interfere with our diners. We don’t turn the music up until late night, around midnight, after the diners are gone,’’ says Yonjan. ‘‘Also, the fact that OM has two levels makes it very easy to separate the lounge experience from the dining room. Now that the DJs are here, actually people are staying after dinner, too.’’

Anise (1 Kendall Square, Cambridge. 617-577-8668), a chic Sichuan restaurant in Kendall Square, is another spot that has emerged with the idea of combining dining and nightlife. However, Shine (617-621-9500. shinecambridge.com), the nightclub in the basement directly below Anise shares only its bathrooms. The two have completely separate ownership, but being so close together, there’s some cross-pollination of clientele.

Anise has a lovely bar and lounge area that stays open until 12:30 a.m., offering gorgeous cocktails courtesy of barman Frank Reardon, who’s also a genial host as well as master mixologist. Just like such wonderful concoctions as the perfectly muddled, fruity Brandy Cobbler, Anise’s food is well priced and perfectly executed.

While the restaurant’s interior is a mix of warm, earthy oranges and reds, Shine has a slick, white and neon-lit decor and a South Beach vibe, and it gets pretty wild. Though it was originally only open on weekends, a new license gives the club the opportunity to follow Anise’s suit and, well, shine, every night.

Diva Indian Bistro (246 Elm St., Somerville. 617-629-4963. divabistro.com) and its adjacent Diva Lounge (248 Elm St.) share a similarly sharp contrast in décor and ambience as Anise and Shine. The bistro started as a hip, mid-priced ethnic restaurant, but the owners expanded last spring to include the impressive Diva Lounge.

An archway connects the two, but the scene is quite different. The lounge is spectacular, with a design aesthetic that mixes ‘‘2001: A Space Odyssey’’ sci-fi with Austin Powers’s 1960s kitsch: With mirrors and white Perspex bubbled walls and a pod restroom to the rear, it’s wild, baby! Late on weekend nights, Euro dance and house zap up the scene, and a bar menu of exotic cocktails fuels a young, trendy crowd. It’s quite the opposite of the comfy relaxed bistro and its excellent Indian food, where, earlier, more ambient Middle Eastern techno pop played. Yet the two work seamlessly.

It’s all about balance and maintaining a high standard for both the food and the club side. Redline (59 JFK St., Cambridge. 617-491-9851. redlinecambridge.com), in Harvard Square, has been a popular choice for late-night dancing for five years. Somehow its restaurant side slipped by the dining public’s attention.

‘‘There’s still the perception that we’re a club, and people don’t know we have food,’’ says general manager Vince Conte. ‘‘So we’ve been paying more attention to that.’’ The menu has been fine-tuned to produce medium-priced, and quite accomplished, versions of modern dining staples.

The transition from restaurant to club is the deal clincher with dining customers. ‘‘The food won’t hurt your nightlife, but the nightlife will affect the food aspect if you don’t pay attention to the transition,’’ says Conte. ‘‘There’s a lot of management that goes into the transition of the atmosphere.’’

Conte points out that as much as he wants to please diners, he doesn’t want to do it at the expense of the clubgoers.

‘‘I don’t ever want Redline to be thought of as a full-scale restaurant,’’ he adds. ‘‘I want both sides strong. When it’s done right, one side rarely hurts the other.’’