Terms like "dermatologist-tested" and "dermatologist-approved" on skin-care products can inspire confidence โ€” even when you're not sure what they really mean. The terms have different meanings, ... However, I have yet to understand the geometrical meaning described on the Gathman's lecture, espeically, " infinitesimally " in one direction.

Context Explanation

Obviously, it is impossible that the double point eat out all affine line. However, I do not surely catch why the double points occupies only very small area of affine line.