What are the three types of coupling?

What are the three types of coupling can be classified in several ways depending on field, but in mechanical engineering for shaft couplings that connect two rotating shafts, three main types are rigid couplings, flexible couplings, and fluid couplings. Rigid coupling, such as sleeve or flange coupling, connects shafts tightly without allowing misalignment, used where precise alignment required and shafts need to act as one, but does not accommodate misalignment. Flexible coupling, such as jaw coupling, spider coupling, Oldham coupling, and universal joint, allows some angular, parallel, and axial misalignment and dampens vibration, protecting machinery, most common type in general industry. Fluid coupling or hydraulic coupling transmits power through fluid, allowing smooth start and overload protection. In plumbing, three types of coupling are slip coupling for repair, compression coupling for joining different materials, and threaded coupling for screwed connections. In electrical, three types are direct coupling, capacitive coupling, and inductive coupling. In relationship context, three types could be casual, serious, and open. But for engineering exam, most common classification is rigid, flexible, and fluid or Oldham as three types of shaft couplings.