A spherical ground joint, a cup, is a type of laboratory glassware used for connecting two pieces of glassware together. It consists of a spherical glass piece with a ground glass surface and a cup-like shape that fits into a matching socket, forming an airtight seal.

The cup design provides a secure hold for the connected glassware and helps to prevent accidental disconnection. This type of joint is commonly used in chemical experiments and is favored for its ease of use and reliability. The ground glass surface provides a secure, leak-proof seal, ensuring that the experiment is carried out safely and effectively.

The spherical ground joint, cup, is often used in laboratory setups where a secure hold and a leak-proof connection are necessary.