It's only a single. It's a clamp-on, supposedly for the handlebars, but It kept slipping, so I repositioned the clamp and put it on the seat post. It wants to slip there, too, but the rim of that stainless bottle will hook over the edge of the cross piece on the back of that seat.

http://www.amazon.com/TOPCABIN-Adjus...er+bottle+cage