What is kinematic pair

Kinematic pair :

A kinematics pair of simply a pair is a joint of two kinematic links that have relative motion with respect to each other. 
When two links in a machine are in contact with each other, they form a pair. Each individual links of a mechanism form a pairing element. 

A degree of freedom of kinematic pair is given by the number of independent coordinates required to completely specify the relative movement between the pair of two links.

Types of kinematic pair :

Kinematic pair can be classified according to :

• The nature of the contact.
• The nature of mechanical constraint.
• The nature of relative motion.

Kinematics pairs according to nature of contact :

• Lower pair 
• Higher pair 

Kinematic pairs according to the nature of mechanical constraint :

• Closed pair 
• Unclosed pair 

Kinematics pair according to nature of relative motion :

• Sliding pair
• Turning pair
• Rolling pair 
• Screw pair
• Spherical pair