Applied Mechanics and Graphic Statics
The angle of friction is:

The angle between the normal reaction and the resultant of normal reaction and limiting friction
The force of friction at which the body is just about to move
The ratio of the friction and the normal reaction
The force of friction when the body is in motion

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

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

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