Which packaging innovation is credited with transforming the romance novel industry in the 1970s, helping to establish it as a mainstream commercial powerhouse?

Product packaging in publishing has evolved dramatically over the decades, with several innovations transforming how readers interact with and select books. From dust jackets to special editions, publishers have continuously experimented with ways to make their products stand out on crowded bookstore shelves. This poll tests your knowledge about a significant packaging innovation that revolutionized a specific segment of the literary market and dramatically boosted sales in that category.

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The clinch cover (featuring embracing couples with the female character's clothing partially displaced)
The foil-stamped hardcover (with metallic accents highlighting the title and author name)
The French flap paperback (with extended cover flaps resembling hardcover dust jackets)
The die-cut window cover (with physical openings revealing part of the first page)