Which innovative product marketing strategy helped Keuffel & Esser dominate the American slide rule market in the early 20th century?

Before calculators and computers became ubiquitous, slide rules were essential tools for mathematicians, engineers, and scientists. These analog calculating devices dominated the market for centuries, but their commercial journey is as fascinating as their mathematical applications. This trivia question explores a pivotal moment in the product marketing history of mathematical tools that changed how these instruments were positioned in the academic marketplace.

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Creating specialized slide rules for different professions (engineering, chemistry, etc.) with targeted marketing campaigns for each field
Introducing a subscription-based rental program allowing universities to provide students with the latest models each semester
Developing the first plastic slide rule, marketed as 'unbreakable' with a lifetime guarantee that revolutionized student sales
Partnering with textbook publishers to include K&E slide rule advertisements and instruction manuals in mathematics textbooks