Ever catch yourself zooming in on Kate Middleton's photos just to admire her perfect eye makeup? You're not alone! Her eyeliner has become a subtle signature at this point, giving her a sophisticated look for every occasion. And finally, we learned that this popular eyeliner is her secret to getting the most flawless look. According to Woman and Home, the Princess of Wales swears by the Lancôme Artliner Liquid Eyeliner - a $34 find that creates a sharp, eye-defining look. The liner has been a beauty classic for decades for its smooth application and foolproof precision.