Copies of Vogue Italia are displayed at Vogue Editorial Office on December 10, 2010 in Milan, Italy.
Copies of Vogue Italia are displayed at Vogue Editorial Office on December 10, 2010 in Milan, Italy. - 

When it comes to the heavyweight championship battle between all the fashion magazines, the big fight happens in September.

"September editions" are traditionally the biggest books of the year -- Vogue's 2012 edition weighed in at 4.5 pounds and was the bane of every mail carrier's existence.

Brands from J Crew to Prada bank on the September editions to market their fall lines. But magazine ad pages can also be a good indicator of how business is booming (or busting) in other parts of the economy, beyond apparel and beauty. They can provide insight into trends in sectors such as technology, travel and entertainment.

Magazine Ad Pages | Create infographics

(Data Sources: 1, 2, 3, 4, 5, 6, 7, 8)

Vogue continues to be the industry's leader, with 665 pages for advertising in its September 2013 issue. It's the most for the magazine since 2007, which had 725 pages of ads.

Fashion magazine ad pages have grown steadily over the past five years, but the rest of the magazine business is following the supermodel look: Thinner and thinner.

Ad buys for all magazines dropped about 5 percent in the first half of this year, as more companies moved their ads toward digital platforms.

As for the future of advertising in fashion? That's starting to go digital, too. Oscar de la Renta announced that their fall ads would debut on their official Instagram, and not in the September issues.

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Follow Raghu Manavalan at @@RaghuNotRagu