Let’s say that there are two negas of the hair or beard one inside the other with a line of hair separating them. If a gap appears in one place, the inner one is unclean; in two places, it is clean. The gap must be the space of two hairs. If there’s a gap in one place the size of a split bean, he is ritually clean.
One who has a nega of the hair or beard with yellow hair is ritually unclean. If black hair later appeared, he is ritually clean; this is so even if the black hair subsequently disappeared. Rabbi Shimon ben Yehuda says in the name of Rabbi Shimon that once any nega of the hair or beard has been purified, it can never again be impure. Rabbi Shimon says that any yellow hair that has been purified can never again be impure.