Ball and socket, or spherical joints have spherically ground mating surfaces. They offer more flexibility in alignment, but must be held together by a clamp. Their designations are similar in form to the taper joints, but the first number is the major diameter of the sphere of the ground part, and the second is the diameter of the bore. A 28/15 joint is 28 mm in diameter at the largest point of the ground section, and the bore is 15 mm. This would mate with a 28/12 joint, although there might be an impedance mismatch if the flow through were critical.
The clamps come in a variety of forms; the most common are metal pinch clamps. There are also plastic clips.