Stoffregen TA, Chen F-C, Yu Y, Villard S. Stance width and angle at sea: effects of sea state and body orientation. Aviat Space Environ Med 2009; 80:845–9. Background: On land, the distance and angle between the feet in side-by-side stance tend to have characteristic values. Ship motion mandates changes in the control of stance, but there have been no direct assessments of how stance width and angle are controlled at sea. We predicted that participants would adopt a wider stance at sea. Method: On two ships, we measured experienced crewmembers’ stance width and stance angle during quiet stance when facing forward and when facing athwartship. Each ship was 84 m long and displaced 3500 tons. Measurements were repeated over consecutive days at sea. For one ship, we tested crewmembers on land prior to the cruise. Results: On land, stance width (mean = 19.0 cm) and stance angle (mean = 16.19°) were similar to previous reports. At sea, stance width tended to be greater than on land. When facing athwartship on Ship B, changes in stance width across days were correlated with changes in motion of the ship in its surge axis (r = 0.968). Stance angle at sea did not differ from stance angle on land. Conclusions: The results suggest that preferred stance width is influenced by the fact of being at sea and by the sea state. These variations may be related to the efficiency of postural control on land and at sea. Future research should examine changes in stance width as novices acclimate to life at sea.