This paper gives the background to the seismic torsional provisions of the New Zealand Loadings Code. These provisions attempt to deal with three effects: accidental eccentricity, torsional ground motion, and coupling between torsional and translational modes. The first two effects lead to an effective eccentricity of one-tenth of the width of a building, while the third leads to a parabolic function of the calculated eccentricity: this was obtained by applying existing results for torsional amplification to a number of typical building structures.