carbon paper

  1. very thin paper coated on one side with a waxy, dark-colored preparation, as of carbon: it is placed between two sheets of paper so that the pressure of typing or writing on the upper sheet makes a copy on the lower
  2. paper used in the carbon process

See carbon paper in American Heritage Dictionary 4

noun
A lightweight paper coated on one side with a dark waxy pigment, placed between two sheets of blank paper so that the bottom sheet will receive a copy of what is typed or written on the top sheet.
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