Designers, models, celebrities, journalists, photographers, and trendsetters have spent the better part of the past four weeks sprinting through a rapid succession of tents that showcase the latest fall and winter fashions.

Distributed across an uneven span of two continents, four cities, and hundreds of designers, each week draws some 100,000 guests, with at least another 2000 -3000 taking care of lights, cameras, and actions that make this one of the most important events for the fashion industry each year.

It’s the kind of thing that magician-turned-model Emma Watson might actually wish she’d had a time-turner for.

Born from the complex socioeconomic tides of the WWII, the first-ever Fashion Week, then named Press Week, was held in New York as a way of distracting stateside consumers from the temporary inaccessibility of French fashions, which, in 1943, were considered the du jour à la mode. New York sought to showcase its own domestic designers, hoping that its takes on fashion would help fill the industry’s void of fresh French garments and techniques.

To almost everyone’s surprise, “Fashion Week” became an instant, international success. In less than a decade, stateside styles had earned themselves a prominent place in the features of the world’s most popular fashion magazines.

Much like the popularizing fashions of the United States, the concept of a weeklong fashion industry event soon spread as well. Though we’ve since witnessed the rise of a “Fashion Week” virtually everywhere, the so-called “Big Four” cities of fashion—New York, London, Milan, and Paris—host two major events per year. They showcase the Fall/Winter and Spring/Summer seasons and are held in February and September of each year, respectively.

We’re currently in the home stretch of Fall/Winter Fashion Week 2012; in September, we’ll already have moved on to the collections for Spring/Summer 2013. The hyper-advanced timeline of these shows allow fashion insiders and editors to begin preparing their celebrities, models, and editorials on a consistent schedule for season-appropriate publication.

But the “premature” timelines also let mainstream fashion vendors (Forever 21, Zara, H&M, Urban Outfitters, and others) implement their low-cost interpretations of these looks. The likeness of Marc’s, Karl’s, and Diane’s looks are almost immediately transposed onto the mannequins of these stores’ next mass-marketed collections.

Almost every new, major fashion trend can trace its original conception to one of the “Big Four” fashion weeks.

Take, for instance, New York City, home to the season’s first and only major stateside fashion week. Hosting the likes of Ralph Lauren, Calvin Klein, Diane von Furstenberg, Betsey Johnson, Anna Sui, Michael Kors, and the finalists of Project Runway, the event draws 3,500 journalists, showcases 2,500 distinct looks, and causes the on-site consumption of some 1,500 bottles of Imperial Vodka—and that’s still 3,500 fewer than the number of free OfficeMax notebooks designed and distributed for frantic front-row note-taking.

Just as each designer developed a distinct reputation for a certain kind of structure or aesthetic, so too does each Fashion Week culminate in several shared trends and ideas. Chanel and Vuitton are established classics, and many “classic” brands—such as Ann Taylor and White House/Black Market—look to them for for that fiftieth tailored jacket, now nothing less than absolutely necessary. Likewise, sportswear brands like Lacoste and Ralph Lauren are predictably preppy; elements of their designs are also subtly adopted into the lookbooks of mainstream brands like Hollister and Abercrombie & Fitch. Elements of von Furstenburg, Jacobs, Rodarte, and Chloé find themselves in Zara, Forever 21, Topshop, and Urban Outfitters.

So, what thematic elements can we expect to be purchase (or even see in stores) this upcoming autumn and winter season?

Darker, muted colors—especially deep pastels, though there does seem to be a dominance of grayer tones above the standard, basic black.

Neutral shades that mimic natural skin tones (like khaki, nude, or brown) paired with neon accents—a bright yellow collar, a lime shirt under a khaki cardigan, or hot pink socks underneath light brown oxfords.

Loose silhouettes.

Minimal, simple, subtle accessories.

These aren’t just vague vogues, either—the trends could have just as easily been oversize bags, metallics, or poodle skirts. Though concepts certainly do change, the ideas themselves constantly turn from a high-profile showcase to a high-profit stockroom.

Of course, there are no hard-and-fast rules, and exceptions are to be expected. Muted colors? Not for Burberry Prorsum or Jeremy Scott. Loose silhouettes? Not for Emilio Pucci.

Perhaps the most curious part of this entire process, is its effect on mainstream consumers. No one needs leggings, skinny jeans, cardigans, plaid shirts, ombre hair, glitter nail polish, combat boots, infinity scarves, or high-low skirts…but we buy them. Because they look good. Because they look different.

But the most important reason? Because they’re there—because of Fashion Week.

Imagine that: a trickle-down theory that actually works.