We analyze the transmission properties of double negative metamaterials (DNM). Numerical simulations, based on the transfer matrix algorithm, show that some portion of the electromagnetic wave changes its polarization inside the DNM structure. As the transmission properties depend strongly on the polarization, this complicates the interpretation of experimental and numerical data, both inside and outside of the pass band. From the transmission data, the effective permittivity, permeability and refractive index are calculated. In the pass band, we found that the real part of permeability and both the real and the imaginary part of the permittivity are negative. Transmission data for some new structures are also shown. Of particular interest is the structure with cut wires, which possesses two double negative pass bands.