The Sisterhood of the Traveling Pants

The Sisterhood of the Traveling Pants (Often shortened to TSOTP) is a series of five bestselling young adult novels by Ann Brashares. Released by Random House, the novels tell the continuing story of four young girls who acquire a pair of jeans that fit all four of them perfectly, even though they are all different shapes and sizes. The four main characters are Lena Kaligaris, Tabitha "Tibby" Rollins, Bridget "Bee" Vreeland, and Carmen Lowell. Carmen delivers the opening prologue and epilogue in first person and tends to play the role throughout the series of keeping the group of friends together.
The series begins in Bethesda, Maryland with the four girls beginning the summer prior to their junior year in high school, and then follows them through four consecutive summers, finally ending with the summer break following their freshman year of college. During this time the girls develop in various ways, but their ultimate goal is to learn to become individuals whilst maintaining their childhood friendship that makes them whole. A spin-off novel was released in 2011 and picks up about ten years later, as the girls are about to turn 30.
Adaptations.
The first book was made into a movie in 2005, starring Alexis Bledel as Lena, Amber Tamblyn as Tibby, America Ferrera as Carmen, and Blake Lively as Bridget. It drastically altered the plotline following Lena and made her story the exact opposite of the book, omitting the presence of her sister, Effie, on her trip to Greece, and changing the development of her relationship with Kostos. It does, however, stay quite close to the stories of Carmen, Bridget and Tibby. Bridget's twin brother is omitted from the film, making her an only child.
"The Sisterhood of the Traveling Pants 2", the sequel to the original movie, was released on August 6, 2008. It mostly follows the fourth book, but also includes Bridget visiting her grandmother, as in the second book, Lena attending her Bapi's funeral, also from the second book, and Carmen's mother giving birth to a boy, from the storyline of the third book. It veers quite dramatically from the books, most likely due to the pressure of compressing three books into one movie.
A film based on the book novel "Sisterhood Everlasting" has been announced and will be produced by Alloy Entertainment.