Sketch Constraints
Apply geometric relationships such as tangency and perpendicularity to sketch curves.
As you create geometry, many sketch constraints are automatically applied. For instance, if you draw a line that is nearly horizontal, a horizontal constraint will be created on that line automatically, making it precisely horizontal for you. When you modify a sketch, the constraints ensure that the intended relationships between the sketch entities remain intact.
Apply or Remove Sketch Constraints
Apply sketch constraints to geometry to ensure that the intended relationships between sketch entities remain intact, or remove sketch constraints to allow more freedom to create freeform objects.
Constraint  Description 

Fixed 
Lock the position of any sketch curve. When a line is fixed, you can still move the end points to change the length. The end points of a line are separate entities and can be fixed independently. Note: To delete all fixed constraints,
rightclick on the constraint and select
Remove Fix.

Horizontal 
Lines are constrained to be horizontal. You can also use this contraint to align points. 
Vertical 
Lines are constrained to be vertical. You can also use this contraint to align points. 
Midpoint 
A point is constrained to be the midpoint of a curve. 
Coincident 
A point is constrained to be coincident with another sketch entity. 
Collinear 
Lines are constrained to lie along the same straight line. 
Parallel 
Lines are constrained to be parallel. 
Perpendicular 
Lines are constrained to be perpendicular. 
Tangent 
A line is constrained to be tangent to an arc, or an arc is constrained to be tangent to another arc. 
Concentric 
Arcs or circles are constrained to have the same center. 
Equal 
Lines are constrained to have the same length. Arcs and circles are constrained to have the same radius. 
Offset 
A sketch curve is offset from another sketch curve. Note: This constraint cannot be applied
from the sketch constraint context menu.

Symmetric 
Two curves are constrained to be symmetric about a construction line (e.g., mirrored curves). 
Select Other  This is not a constraint. Select this option when selecting overlapping sketch entities to specify which sketch entity you wish to select. 
 To hide all constraints, deselect Show constraints under Sketching in .
 If your horizontal constaints appear vertical and vice versa, use the Align Axes option in the context menu to align the sketch axes so that the xaxis is horizontal and the yaxis is vertical.
 Select Delete All Constraints from the context menu to remove all constraints from all sketches in the model.
 Select Remove Fix from the context menu to remove all fixed constraints in the model.