Which packaging innovation revolutionized dictionary usage in the 1970s, making language reference more accessible in everyday settings?

In the world of language reference materials, packaging has evolved dramatically over the decades. Dictionary publishers have innovated not just in content but in how their products are physically presented to consumers. This trivia question explores a significant packaging innovation that changed how we interact with dictionaries and language reference materials in everyday settings.

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The introduction of the thumb index for rapid letter navigation
The development of the paperback pocket dictionary with protective vinyl cover
The addition of transparent acetate sheets protecting pronunciation guides
The implementation of color-coded page edges for different language sections