The 2010 Tour de France, the biggest event of the entire cycling year, starts with a stage so short and seemingly inconsequential that they don't even give it a number. It's the Prologue, a 9 kilometer romp around old Rotterdam, that gets the three-week race officially underway. What? Why? Let's delve in...
The What
A prologue is kind of a stage but not really. Tour de France prologues are generally between 5 and 9km in length. Historically the Tour has used prologues most years since (arguably) 1967. At first a prologue-esque time trial was considered stage 1a, and when that wrapped up the peloton would go back to the start line for a mass-start stage, called 1b. Riders don't like doing two stages in a day, so in 1971 the Tour started running the prologue on its own, and by 1974 the word "prologue" began appearing in place of a stage number. If the prologue was Saturday, then stage 1 was Sunday.
Some anomalies: two years ago in a fit of nostalgia the Tour started with a normal road stage. This was in Brittany, a hotbed of cycling tradition, so the locals didn't mind passing on the prologue gimmick. On a few occasions the Tour has used a longer time trial to open accounts, like in 2005 when Lance Armstrong passed Jan Ullrich on the road -- a rather embarassing way to start a Tour. Anyway, if the stage cracks 10km, the Tour will call it a time trial and give it a stage number. The other grand tours -- Giro d'Italia and Vuelta a Espana -- have used team time trial prologues on occasion -- a great spectacle IMHO. And then there was 1988, when the Tour used a 1km "preface" won by sprinter Guido Bontempi. Thankfully that was a one-time experiment.
The Why: Classifications
One simple reason for a prologue is to make it possible to hand out jerseys. Often the Tour starts with flat stages, at the end of which riders finishing in a big bunch are all given the same finishing time. Since the yellow jersey is given to the guy with the fastest accumulated time, the Tour would have to use tie-breaking rules to award the famed yellow jersey, the maillot jaune. Not the dignified process the maillot jaune deserves, and without a prologue, these time ties could go on for several days. With a prologue, a nice (and basically inconsequential) order is established. Also, since it's a time trial, the stage and first yellow jersey is often won by a non-sprinter, which is nice since the sprinters get most of the spoils for the next several stages. Share the love, they say.
The Bigger Why: $$
A much more important reason for a prologue is to reward the city which hosts the Tour's opening, the Grand Départ. Throughout the Tour cities are chosen to host a stage start or finish not randomly but because they paid the Tour de France for the honor. The Tour is not government-run, it's a privately owned event (currently in the hands of the esteemed Amaury Sports Organization, or ASO) and it needs to pay all of its bills. ASO raise revenue by advertising, selling broadcasting rights, hosting VIP events, and selling stage starts and finishes. The finale has to be in Paris and the decisive stages have to be in the Alps or Pyrenees mountains, but the rest of the stages are for rent.
The biggest stage-hosting opportunity is now billed as the Grand Départ. I hadn't heard this term until recent years, but it's long been an honor to host the first weekend's events (gets talked about for a good six months prior), and has evolved into something of a spectacle. Take 2007, when the Grand Départ came to London. I can't find the bid amount (help!) but London sought the start of the Tour to bring attention to their bid to host the 2012 Olympics. The mayor of London also used the event to announce new investment in cycling facilities in the city. The stage itself wound through famous sections of venerable London and made for a truly spectacular day, well beyond what you'd expect from a mere 6km of racing.
Had the 2007 Tour begun with a normal road stage, the experience would have been quite different. Race starts are funny things. They consist of a lot of pointless milling around, with fans spying idle bikes next to closed team buses and local dignitaries sipping the drinks of choice in a "luxury tent" while a guy with a mic interviews whoever he can grab: the mayor, a passing rider, etc. Finally the riders appear, with the biggest names waiting til the last possible moment. They sign in, mount their bikes, the gun goes off, and within a few minutes the whole thing is gone. A few hours' later, when the stage has been dismantled, you'll wonder if you dreamt the whole thing.
Well, that would have been a poor return for London's investment, so instead they got a race which went around the city all day long. Fans got to see riders one by one, in a fast-moving gala showcasing the people and what they can do on their machines. TV cameras on motorbikes wound around the city's spectacular streets all day long, beaming images of London to every TV you can think of. The party lasted for hours on end, before, during and after. And the next morning London got one last glimpse of the Tour as they sent off the peloton in a start like I just described above. Now THAT's value, and the Tour can sell a prologue for vastly more than it can sell a typical stage start.
A Warm-Up: The Not-Why
One last note -- riders don't particularly need a prologue. They may enjoy the scene, which buzzes a lot more than the usual road stage, but their bodies are conditioned to long, hard efforts, not idle warmups and a 15-minute sprint. From a racing standpoint, prologues are tolerated at best. Except by the guys who can win and wear yellow for a few days. They're licking their lips.