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.
Next to the Control Panel, a legend is displayed indicating by color which sketch curves are Underdefined, Fully Defined, Overconstrained, Fixed, and Imported.
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. 
Construction  This is not a constraint. Select this option to convert a sketch curve into construction geometry, which is used as a reference to line up or define other geometry. Note: To hide construction geometry, rightclick, and
then deselect Show construction
geometry.

Horizontal 
Lines are forced to be horizontal. Points are forced to have the same y (with respect to the sketch plane). 
Vertical 
Lines are forced to be horizontal. Points are forced to have the same x (with respect to the sketch plane). 
Parallel 
Lines are forced to be parallel. 
Perpendicular 
Lines are forced to be perpendicular. 
Tangent 
Curves are forced to be tangent. 
Coincident  Geometry is forced to be coincident with another geometry. 
Concentric 
Centers of arcs and circles are forced to be coincident. 
Collinear 
Lines are forced to be coincident with the same straight line. 
Equal  Lines are forced to have the same length. Arcs and circles are forced to have the same radius. 
Midpoint  A point is forced to be coincident with the midpoint of an open curve. 
Symmetric  Two curves are forced to be symmetric about a construction line (e.g., mirrored curves). 