On the trailer with the mast up. When the mast is up, I stretch a rope from the bow pulpit to the backstay to act as a "ridgepole" to hold the canvas up. When the mast is down in the trailering position, the mast acts as the ridegepole. When the boat is on the trailer I use bungees to hold it down. It can be used on the water too, with weighted bags clipped onto the bottom edge over the cover.