Mendel was the first to use double letters for his hybrids—indicating that he understood that a hybrid carried two differentiating character traits, one of which was hidden from view until it chanced to reappear in subsequent generations in a different combination. This made it seem as though Mendel knew that each individual would possess two of these traits, or Merkmale, one from each parent. Yet in the pure-breeding plants, either pure yellow or pure green, he used only a single letter to represent the full complement.