Command call:
Icon 
Ribbon 

Draw > Constraints > Constraint > Middle Workplane > Constraints > Constraint > Middle 

Keyboard 
Textual Menu 
<SC>><M> 

Parameters window of the <SC> Constraint command 

<M> 
Middle 
The constraint is created by three points, or by two line segments and a point. The points can be any characteristic points of graphic lines.
When defining constraints by three points, we define the following interrelations. The first given point P1 and the second given point P2 define a segment. A straight line normal to the segment passes through its center. A third point P3 will lie on the normal line. All the constructions in the figure are an explanation of the principle of setting the Middle constraint by three points. When creating a constraint, these constructions are not shown.
Another option for setting a constraint is by two segments and a point.
The first given segment L1 and the second given segment L2 define the direction of the straight lines, which form an angle when crossing. If we draw a bisector of this angle, then the point P3 must always lie on it.
It can be explained otherwise. If perpendiculars are dropped from point P3 to segments L1 and L2, then the perpendicular segments will have equal lenght. The second explanation is also true in the case when the constrained segments are parallel. All additional constructions in the figure are an explanation of the principle of setting the Middle constraint by two segments and a point. When creating a constraint, these constructions are not shown.
When creating a Middle constraint, if other constraints do not prevent movement of objects, then the midpoint gets moved. If the midpoint is fixated, then the object selected second gets moved. If the second object is fixated too, then the first object gets moved. Lengths of segments are preserved upon moving. If the second object is a segment with a single fixated point, then its free points get moved along with the first segment, therefore length of the second segment gets changed, while length of the first segment is preserved. If both segments have one fixated point each, then free points of both segments get moved, therefore lengths of both segments get changed.
Icon of constraint is displayed near middle point.
Examples
Two Lines Perpendicular to each other with Intersection in the Middle.
Let two arbitrary segments be given. Let's make it so, that their point of intersection is in the middle for each of them. First, we fix the length of the segments by the constraint Fixed length. Next, in the ribbon, select the constraint Middle and with the help of select the two endpoints of the first segment and the midpoint of the second.
We finish the input of constraint objects with the help of or <Ctrl> + <Enter>. The second segment is moved, so that its midpoint lies on the perpendicular from the middle of the first segment. Next to the midpoint of the second segment, an icon of the Middle constraint appears.
Similarly, we create the second Middle constraint: with the help of we select two extreme points of the second segment and the midpoint of the first segment, then we finish creating the constraint. The first segment is moved so, that its midpoint lies on the straight line normal to the second segment, and passing through the center of the second segment. Next to the midpoint of the first segment, an icon of the Middle constraint appears.
Now it is necessary to make the segments perpendicular. To do this, we use Perpendicular constraint. The segments moved in such a way that they became perpendicular to each other, and due to two Middle constraints the intersection point divides each of the segments in half.
The Axis sets the Middle Line between the Two Segments.
Let three line segments be given. One of the segments is an axis and uses the appropriate type of graphic line. We position the segments vertically using Vertical constraint (select the constraint and with the help of select three segments). Then we make segments' lengths equal using the Equal length constraint. Next, align segments with the help of the Horizontal alignment constraint (choose the lower points of the segments with the help of ).
Then we select the constraint Middle in the Ribbon, with the help of we specify two continuous segments and a point on the axis segment. Finish input with or <Ctrl>+<Enter>. An icon of the Middle appears next to the point on the axial line segment.
When editing, the segments, due to constraints, will always remain the same length, at the same height, always vertical, and the segment of the axis will always be in the middle.