Based in Edinburgh's swanky New Town, the Assembly chain rolls out a high-profile annual programme across a prestigious portfolio of venues. TV stars, international theatrical troupes, and musicals tuning up for their West End run - all have made their Edinburgh appearances here. And, while you might see a show you don't like at the Assembly... it's unlikely you'll find a truly bad one.

Fans of the Edinburgh Festival have a genuine fondness for the Gilded Balloon. It's not just the venue's impressive record for "discovering" new comedians, nor just that they - alone among the major venues - are actually run from Edinburgh. No, this special relationship was forged in flames, on the cold night when the catastrophic Old Town Fire destroyed the GB's original Cowgate home.

The Pleasance chain spans just two venues - but oh, what venues they are. Every August, more than a quarter of a million people call into the vast outdoor campus of the Pleasance Courtyard, or the palm-shaded atrium of the Pleasance Dome. Between them, the two sites sport 20 performance spaces, 10 bars, and a packed line-up of almost 200 shows.

The newest and edgiest of the major venue chains, Underbelly is the Fringe's teenage son - rebellious, unkempt, and just a little gawky. There's a thrown-together feel to its spray-painted signs, and shabby-chic charm abounds in its ramshackle main venue. But don't be fooled: in just eight years, Underbelly's raced from a standing start to become one of the best-known brands on the Fringe.

Adam House, landmark headquarters of the C Venues chain, is solid, understated and purposeful. You can say the same about the operation itself: everything about C radiates calm efficiency, from the airport-style information screens to the "last calls" for shows. While bigger-name rivals revel in controversy, C just gets on with the job.

The smallest of the Fringe's major chains, Sweet's growing network now runs to three venues across the south side of Edinburgh. Sweet's eclectic programme's built mainly from small-scale amateur productions, but otherwise it's hard to pin down: crossing most genres, Sweet even runs a workshop or two to encourage future Fringe stars.

One of two major chains promoting free shows on the Fringe, comedy club operator Laughing Horse runs eight venues under the "Free Festival" banner. As the name suggests, all the shows are free to attend, with only a voluntary collection at the end. It's a concept that's taken off in recent years, and the city-spanning Laughing Horse network even now includes its own small mega-venue - three spaces in city-centre nightclub Espionage.

Named after founder Peter Buckley-Hill - comedian, musician and joyous self-publicist - PBH's Free Fringe is the product of an acrimonious split from rival operator Laughing Horse. Whatever the truth of the much-publicised dispute, few would challenge PBH's claim to be the true inventor of the Free Fringe model - where venues and performers offer their services for free, in exchange only for bringing a crowd to the bar.