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* *Constraint* is a set of two or more [joints|kb:Joint] which are _constrained_ such that their displacements relate. Constraints may be used to model 1). rigid-body behavior, where joints translate and rotate together in a rigid connection; 2). equal-displacement behavior, where displacement along certain DOF are equal; and 3). symmetry / anti-symmetry conditions. Constraints enhance computational efficiency by reducing the number of equations necessary for solution.

* *Body constraint* is applied to a set of joints such that they translate and rotate together as a rigid body. By default, [rigid behavior|kb:Rigid behavior] is applied to all degrees of freedom (DOF), though users may specify body constraints for only certain DOF. Body constraints 1). simulate rigid connections; 2). connect portions of a model defined by separate meshes; and 3). connect [frames|kb:Frame] to [shells|kb:Shell].

* *Weld* is useful for connecting parts of a model defined by different meshes in that it generates multiple sets of body constrains between joints which are coincident, meaning located within a certain distance of one another. This range is known as the tolerance. Only joints within the weld are checked for coincidence. 
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The [_CSI Analysis Reference Manual_|doc:Analysis Reference Manual] (Chapter 5 > Page 49) provides additional information on these three, and the many other, constraint types available.

* *Restraint* concerns only the behavior of an individual joint. This is different from a constraint condition, which is associated with the relationship among a set of joints. Restraints must be applied to certain DOF when joint displacement is known. This displacement may be zero, such as with support points, or non-zero, such as with support settlement. The force necessary to produce a specified restraint displacement is known as the _reaction_, and is generated during analysis. Restraints should also be applied when stiffness is zero along certain DOF, such as with the out-of-plane translation and in-plane rotation of a planar frame. Additional information is available in the [_CSI Analysis Reference Manual_|doc:Analysis Reference Manual] (Chapter 4 > Page 34)

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