Constrains one line or polyline segment to be collinear with another one, which means they lay in the same line. (This command is equivalent to using the Collinear option of the GeomConstraint command.)
Note: The second entity you select is straightened out to be collinear to the first.
Accessing the Command
command bar: gccollinear
menu bar: Assembly  2D Constraints  Collinear
ribbon: Parametric  2D Constraints  Collinear
toolbar: 2D Constraints 
: GcCollinear
Prompts you in the command bar:
Select first entity or [Multiple]: (Select a line or polyline segment, or press M)
Select second entity: (Select another line or polyline segment)
Command Options
Option 
Description 
Select first entity 
Select a line or polyline segment. 
Select entity to make collinear to first 
Select multiple lines or polyline segments; segments of the same polyline are accepted, which is straightens out the polyline. 
Multiple 
Makes more than two entities collinear; you can pick only one entity at a time. Prompts you:

After you place the constraint, icons appear near the entities indicating the type of constraint. If you do not see them, then use the ConstraintBar command to display them. To remove a constraint, click the tiny x on the constraint bar.
Related Commands
GeomConstraint  applies geometric relationships between entities and/or points on entities.
Dimconstraint  applies a dimensional constraint to an entity or between constraint points on entities; converts associative dimensions to dynamic dimensions.
GcCoincident  applies a coincident geometrical constraint to two points or constrains a point to an entity.
GcConcentric  constrains the center points of circles, arcs, ellipses or elliptical arcs to coincide.
GcEqual  constrains arcs and circles to the same radius, or lines to the same length.
GcFix  constrains points and entities at a fixed position.
GcHorizontal  constrains lines or pairs of points to lie parallel to the Xaxis of the current coordinate system.
GcParallel  forces two lines to be parallel to each other.
GcPerpendicular  constrains two lines to lie perpendicularly to each other.
GcSmooth  forces a spline to maintain fluid geometric continuity with another spline, line, arc or polyline.
GcSymmetric  constrains two entities or points to lie symmetrically with respect to a selected line.
GcTangent  constrains two entities to maintain a point of tangency to each other or their extensions.
GcVertical  constrains lines or pairs of points to lie parallel to the Yaxis of the current coordinate system.
ConstraintBar  controls the display of the constraint bar of entities.
Delconstraint  removes all dimensional and geometrical constraints from an entity.