The technique Gaudí used to design the church was to hang little bags of birdshot from strings. Gravity would pull these bags downwards, giving even weight distribution and stretching the strings to form a model structure, thus showing him the shapes and angles his pillars would need to be. By photographing the model, then inverting the photograph, Gaudí could then see the model as it should look. A replica of his model for the crypt is in the museum under the Sagrada Família in Barcelona. (Wikipedia)

Only the crypt was completed; this is what it looks like today: