(InStyle.com) -- 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" ($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.
Jennifer Lopez -- "The plunge," Donatella Versace (2000)
"This is probably the most famous red carpet dress ever. The irony is that it's not exactly a dress. The dress is attached to a brief. It's really a onesie, which is funny because it's a childlike design for the sexiest dress that's ever been on a red carpet. At this point, everyone thought Jennifer was a sexy girl but this turned her into a bombshell -- Hollywood's biggest bombshell."
Catherine Middleton -- "Wedding gown," Sarah Burton for Alexander McQueen (2011)
"What Kate shrewdly did -- and what she continues to shrewdly do -- is that she managed to figure out a way to balance how to look like a 21st century woman and still hold up the integrity and the dignity of the royal family. It was exactly the right dress to wear. Everybody had a crush on her at the time, but when you saw this, you just melted and fell in love. It was a home run, bases loaded with two outs in the bottom of the ninth."
Elizabeth Hurley -- "Safety-pin gown," Gianni Versace (1994)
"This dress is the fashion equivalent of the shot heard around the world -- one day some people casually know her, and by the next morning one dress made this woman world famous. The funny part is that Elizabeth didn't actually pick the dress. Gianni Versace just put it in a box and sent it to her because he thought it was what she should wear. The popularity of this speaks to our fascination with fashion, beauty, and sex appeal. Let's face it -- it's about as sexy a dress as it gets."
Diana, Princess of Wales -- "Revenge dress," Christina Stambolian (1991)
"This is a great example of one-upsmanship. Diana had bought this dress while she was still Princess of Wales, and she felt it was too sexy. But after her separation from Prince Charles, and after the fairytale crashed, she found out on the very night that she was going to attend an event at the Serpentine Gallery in London, that Charles was about to announce that he had always loved Camilla Parker Bowles. Well, out came the dress and the highest shoes she could find. And who wound up on the front page the next day? It was Diana, rocking out."
Uma Thurman -- "Lavender Oscar dress," Miuccia Prada (1995)
"Now we look to Miuccia Prada as one of the most influential people in the fashion world. Yet in 1995, Prada was still mainly known for nylon knapsacks and quilted bags. This dress changed the entire perception of the brand. First of all, the color is exquisite. The gentle layers of chiffon and the hand on this dress is so delicate. And to choose Uma Thurman to dress, she is basically an ethereal, unearthly beauty. It made everyone say, 'OK, maybe we should start looking to Prada as a place for clothes.'"
Nicole Kidman -- "Chinoiserie gown," John Galliano for Dior (1997)
"This dress literally raised the bar of what we expect to see at red carpet events. This is the first time in a long time that an haute couture gown had been worn onto the red carpet. She and her stylist also borrowed jewelry, which was unconventional at the time. So the dress was unlike anybody else's, the jewelry was unlike anybody else's -- Nicole didn't look like other movie stars."
Kate Moss -- Heather-gray shift," Narciso Rodriguez for Cerruti (1997)
"This dress shows you the power of truly minimalist design. It's not just a shift -- there are a whole series of seams that go up the back of the dress that contour to Kate's body. Narciso's awareness of the female body is uncanny. Remember, this is Cannes. People dress bigger than they do at the Oscars. Then, here come Kate and Johnny, and she shows up looking so simple in a heather gray short dress, and just upstages everybody."
Gwyneth Paltrow -- "Grace Kelly gown," Ralph Lauren (1998)
"This is the moment when Gwyneth became Hollywood's new golden girl. Ralph Lauren has always cited Grace Kelly as his sort of epitome of Hollywood glamour. And basically Ralph wanted to basically give her a Grace Kelly 'The New Princess of Hollywood' moment. Turns out, that's not all she wanted to do with her life. She actually rejected the position that Hollywood offered her. But for this one particular night, you thought she was going to be the successor of Grace Kelly."
Charlize Theron -- "Golden-age gown," Vera Wang (2000)
"Vera cleverly played upon Charlize's old-fashioned glamour and took a modern twist on it. While it looks like iridescent tangerine, there are actually five layers under the dress to create this amazing color. This dress is different because it's a dress that women can relate to. Most gowns on the red carpet are not, and yet this is a dress that is thoroughly relatable, has that inspiration for the '30s and still is incredibly modern now."
Renee Zellweger -- Yellow chiffon gown, Jean Desses (1955; worn in 2001)
"Technically, vintage is anything over 50 years old. Renee's dress is about four years short of being exactly 50 years old, but it's as close as we're ever going to get. And yet looks so incredibly timeless -- its elegance, its sophistication, its ability to flatter remains completely unchanged. This dress shows you the power and the lasting effect of great design."
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