[If] two women made two kav [doughs], and they touched one another, even if they are of the same kind, they are exempt. But when they belong to one woman, a kind with its own kind is obligated, but with a different kind it is not obligated.

What is a kind with its own kind? Wheat does not combine with anything except with spelt. Barley combines with everything, except for wheat. R' Yochanan ben Nuri says: The other species combine with one another.