"Nüshu", or "women's book", is a writing system that was kept secret for centuries. In feudal China – when women were denied an education, bound to their husbands and rarely left their homes – they developed their own language. This written code allowed them to communicate without being understood by men. Nüshu has almost disappeared but today, in the region where it was born, some women are striving to keep it alive. Our correspondents report from Hunan province.