Centerbar buckles are used in belts and other accouterments. The leather folds around the center of the buckle leaving two outer bars. One for the piece to feed under and the second to add as a keeper.
Bought this buckle for a sling I was making for my Brown Bess. I wanted a wider sling to fit better on my shoulder when carrying it in the field and the buckle really ties everything together. Very pleased with how it turned out.