I was sceptical to purchase this outdoor sofa cover . Others I've seen in stores were made of thin plastic . So we've been bungee tying tarps to cover our outdoor seating.  But this is really well made and sturdy.  
It has heavy duty adjustable clips for each leg and cinches along the bottom on both sides .
We like to stack our chairs on top of the sofa in the winter . This has plenty of room to cover it all and stay in place .