The Inca system of writing in khipus, or knotted cords ©The Trustees of the British Museum THE Incas left no doubt that theirs was a sophisticated, technologically savvy civilisation. At its height in the 15th century, it was the largest empire in the Americas, extending almost 5000 kilometres from modern-day Ecuador to Chile. These were the people who built Machu Picchu, a royal estate perched in