With thousands of dresses hitting the red carpet every year, we have to wonder: What makes a dress unforgettable? To get the answer, we turned to Hal Rubenstein, InStyle Fashion Director and author of the new book 100 Unforgettable Dresses, out today ($35 at amazon.com). “An unforgettable dress isn't merely beautiful,” he says. “It’s about the right dress on the right woman at the right time. Ultimately, it alters how others dress the morning after, how we see pop culture, and how we see ourselves.” See 10 of his most memorable looks in recent history in the gallery, from Jennifer Lopez’s plunge dress to Elizabeth Hurley’s safety-pin look. For more, pick up his new book, available in stores now.