Most bike panniers are purchased as a set of two bags. Although it is possible to ride with just one bag, having two makes for a smoother, more balanced ride. There are a few bags that are sold individually, so make sure you are clear on what you are purchasing before actually buying a bike pannier. This is important not just because it’s disappointing to think you bought a set of two only to receive one pannier, but also because bags sold individually may end up being more expensive than ones sold as a set.