The so-called roll center is not a concept well defined for a rolling ship or platform when submitted to a wave field. The present paper discusses and proposes a clear definition. The paper shows a methodology to assess this point and shows that, for regular beam wave incidence on a symmetric body, the roll center is not necessarily located at the line of symmetry of the symmetric bodies. Also shows that the locus of the roll center is frequency dependent. Finally, the paper discusses the limits for low and high frequencies. This investigation uses basic equations of the rigid body kinematics and information for better understanding the complicated roll center. To validate the proposed methodology the paper reports model tests and frequency domain calculations regarding the behavior of the vessel in regular beam wave. A closed form equation for the calculation of the roll center is also proposed. All these results match very well. This is so, despite the very complicated phase behavior with frequency. The paper also addresses the question whether the bilge keels at each board should always be symmetric for a platform that will always operate in beams seas.

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