Much of Gaudi's architecture was inspired by natural forms. Underlying the impression of freedom and flow of his work, it is apparent from his sketches that there is a rigorous understanding of the mathematical structures and proportions at work in nature, such as the Fibonacci series. He also designed the furniture and fittings for his buildings, some of which can be seen in these photos of Casa Mila, La Sagrada Familia, and Parc Guell, all of which are in Barcelona.