We report on extensive experimental and theoretical studies of domain patterns in type-I superconductors. Both superconducting (SC) and normal state (NS) domains are analyzed using magneto-optical imaging. We measure the average width (diameter) of lamellar (circular) domains. The experimental values and predicted equilibrium values are shown to present a significantly better agreement when the screening of the magnetic flux is taken into account. On this basis, we analyze the contribution of the elongation instability of circular (NS and SC) domains on the formation of the intermediate state. The most surprising and specific aspects of the formation of domains are shown to originate from the perfect diamagnetism of the superconducting phase. (c) 2007 American Institute of Physics.