Mechanism Design and Mechanism Dynamics > Mechanism Design > Creating Mechanism Models > Connections and Degrees of Freedom > About Predefined Constraint Sets
About Predefined Constraint Sets
You must choose a predefined constraint set when you place a component in an assembly. A predefined constraint set serves three purposes:
Defines which placement constraints are used to place the component in the model
Restricts the motion of bodies relative to each other, reducing the total possible degrees of freedom (DOF) of the model
Defines the kind of motion a component can have within the mechanism
Before selecting a predefined constraint set, you must understand how the placement constraints and degrees of freedom are used to define movement. You can then select the right connections for the way you want your mechanism to move. 
Each predefined set is associated with a unique set of geometric entities, including points and datum axes.
For detailed information about constraint sets, search the Help Center.