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In page Slide rule:

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All premium slide rules had numbers and scales deeply engraved, and then filled with paint or other resin. Painted or imprinted slide rules were viewed as inferior, because the markings could wear off or be chemically damaged. Nevertheless, Pickett & Eckel, an American slide rule company, made only printed scale rules.[citation needed] Premium slide rules included clever mechanical catches so the rule would not fall apart by accident, and bumpers to protect the scales and cursor from rubbing on tabletops.