A decade ago Chinese-American author Lisa See was researching an article on footbinding when she found a reference to Nushu, the world's only "women's writing". Though the origins were murky, the script revealed a culture of women's relationships and sparked the idea for her novel, Snow Flower and the Secret Fan, the film of which, co-produced by Rupert Murdoch's wife Wendi Deng, is released tomorrow.

After having their feet bound at around the age of seven, girls in Jiangyong County in Hunan province would live indoors – first in the "women's chamber" of their own homes, and later in the homes of their husband's family. To ease their isolation and offer support in their pain, girls from the same village were brought together as "sworn sisters" until their weddings. But a more serious relationship, almost akin to marriage and expected to last for life, could be arranged between two girls by a matchmaker, with a formal contract, if the pair shared enough of the same "characters" (being born on the same day, for example). In See's book she writes: "A laotong relationship is made by choice for the purpose of emotional companionship and eternal fidelity. A marriage is not made by choice and has only one purpose — to have sons."

Women used Nushu – a script unique to the area – to write to their laotongs after they "married out" into different villages. Yet until the 1960s few outside the province knew about it, and no men could read it, says See. "In the mid-60s an old woman fainted in a station," she says. "The police went through her things to see who she was and found a piece of paper with what looked like a code, so she was arrested on suspicion of being a spy." In the midst of the cultural revolution, the experts who finally identified the script were sent to labour camps, not emerging to study the writing until the 80s. Following the success of See's book, the film aims to allow people to learn more.