Black Click Clack Slotted vs Unslotted. Which should you buy? The main difference between a slotted and an unslotted siphon is, a slotted siphon is designed for sinks with overflow. So if you are buying a sink with an overflow included, our recommendation would be to buy the slotted option, and vice-versa, if your sink doesn't include an overflow, buy the unslotted option. We hope this helps in your bathroom shop. Technical Details Type: Basin Colour: Black Thread Diameter: 40 mm Height: 80 mm Material: Brass