Applied Mechanics and Graphic Statics
The characteristic of a couple is:

A couple can be never the balanced by a single force
Algebraic sum of forces, constituting a couple is zero
All listed here
Algebraic sum of moments of forces, constituting a couple, about any point, is same

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Applied Mechanics and Graphic Statics
A point subjected to a number of forces will be in equilibrium, if

Algebraic sum of the moments of the forces about the point is zero
Sum of resolved parts in any two directions at right angles, are both zero
Two resolved parts in any two directions at right angles are equal
Algebraic sum of the forces is zero

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