I'm probably more annoyed than I should be by this common mistake, but I used to be a math teacher and have taught geometry to students from first to sixth grade. I see this blunder everywhere lately: on blogs, YouTube tutorials, and even in beauty books (where are the editors?). It's also in PR pitches for various contour and highlighting products, and it drives me up the wall, so please allow me a moment of crankiness:
When applying foundation and concealer we strive to create a smooth canvas, but that can appear too flat. The face loses some of its depth, and the features appear TWO DIMENSIONAL. Not One dimensional. TWO. Because one dimension means a line. The plane of the face has both length and width. Since we want to re-add some depth we contour and/or highlight accordingly, to emphasize the three dimensional nature of the face.
Class dismissed. I'll be at my desk munching on as apple.
Image via skillsyouneed.com