A boiler design where the diameter at the smokebox end is smaller than at the firebox end. This was done so that the maximum area possible was available for heating around the firebox. A tapered boiler was first introduced in the US in 1850, where it was referred to as the 'wagon top' boiler. The design has the added advantage that the joints between the boiler rings do not require to be formed to provide a lap, but they will provide a natural lap.