Sometimes you need to mount a flat-bottom tripod head on a half-ball style tripod. When that happens, use this adapter and your world will be harmonious once more (we're assuming it was, at some point, harmonious). This particular adapter fits into a 100mm half ball, and gives you a standard 3/8-16 threaded mount on the top. This model also has a very short tie-down thread on the bottom, giving you lots of clearance for low-angle shots.