AutoCAD 2011 significantly simplifies the process of adding geometric constraints to 2D AutoCAD geometry. AutoCAD can infer geometric constraints as you create and modify geometry. A new Infer Constraint button on the status bar enables you to toggle inferred constraints on and off similar to toggling object snaps on and off. This toggle is also available on the Geometric tab of the Constraint Settings dialog box.
Using inferred constraints automatically applies coincident constraints for Endpoint, Midpoint, Center, Node, and Insertion objects snaps. For example, if you draw a circle by snapping its center to the midpoint of a line, AutoCAD automatically applies a coincident constraint between the center of the circle and the midpoint of the line. If you move the circle, the line goes with it. The same powerful functionality applies to editing commands. If, for example, you copy the insertion of a block to the endpoint of a line, AutoCAD automatically applies a coincident constraint between those two points. When you move the line, the block maintains its position at the endpoint of the line.
In addition to creating point-to-point coincident constraints with the previously mentioned object snaps, you can use the Nearest object snap to apply a coincident constraint between a point and an object. For example, if you draw the center of a circle on a line using the Nearest object snap, the center of the circle has the flexibility of moving anywhere along the line including the virtual extension of the line.
The Perpendicular and Tangent object snaps automatically apply perpendicular or tangent constraints between the object being created or edited and the object being snapped to. They also apply a point-to-point or point-to-object coincident constraint if appropriate. Using the Parallel object snap automatically applies a parallel constraint but no coincident constraints are applied since, by definition, the two objects never intersect. When you draw horizontal or vertical lines and polyline segments, inferred horizontal and vertical constraints automatically apply to those segments.