The T-mount system uses T-threads of metric dimensions 42mm diameter and a pitch of 0.75mm to mate cameras to lenses and telescopes as well as other devices.
T is for Tamron, a long-time maker of lenses and accessories to fit many different makes of cameras. They devised the system in the 1950s-1960s to sell their lenses to fit other makes of camera bodies.
The T-mount system consists of four parts: the camera, a T-ring, a T-adapter and lastly a lens or a telescope.
The T-ring fits into a specific brand of camera on one side and has female T-threads on the other side. T-rings are made and sold to fit camera bodies.
The T-adapter fits into a specific brand or kind of lens or telescope on one end and has male T-threads on the other end. T-adapters are made and sold to fit lenses and telescopes, as well as other devices such as bellows.
When screwed together, the matching threads on the T-ring and T-adapter allow a camera to be rigidly mounted to a lens or a telescope.