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When a fluid conducting tube occludes, the surface interface of the fluid becomes a stationary capillary surface spanning the tube. We extend known criteria for existence and non-existence of such a surface, to obtain an occlusion criterion for cylindrical tubes of circular section situated horizontally in a vertical gravity field. We show additionally that if the materials are such that the contact angle is small, tubes of suitable non-circular section can be designed so as not to occlude regardless of gravity.