After selecting your harness, your next choice will be the aircell, often called a "wing" because of how it appears on the back of the diver. Frankly, a very common error when selecting a wing is to choose a size much too big, causing a 'taco effect' that makes managing buoyancy considerably more difficult.
The cylinder size, type and configuration, not lift capacity, is the best guide to selecting the correct wing. While many doubles wing designs can be adjusted for occasional use with singles, keep in mind they are not designed to be used primarily as singles wings and they are not optimal with singles. Realistically, there is no wing perfect for both singles and doubles, or even a wing perfect for all doubles. If the range of cylinder configurations you dive is broad, then the best choice for efficient and comfortable diving may be to own more than one wing. More information is available in our article Suggestions for Selecting a Wing.
