Constraint
Introduction
In FreeCAD the word "Constraint" is normally used to refer to a "rule" to draw geometrical shapes inside a 
Sketch (Sketcher::SketchObject class). A constraint limits the position of a certain geometrical element in different ways, for example, it can specify whether the element is horizontal, vertical, tangent, parallel, perpendicular, coincident with a point, concentric to another object, etc.
There are two big types of constraints:
- Geometric constraints define characteristics of the shapes without specifying exact dimensions, for example, horizontality, verticality, parallelism, perpendicularity, and tangency.
- Datum or dimensional constraints define characteristics of the shapes by specifying dimensions, for example, a numeric length or an angle.
See the information in the Sketcher Workbench for a list of all constraints that can be applied. Some of them apply to lines, some to curves, and some to vertices. See also the basic sketcher tutorial.
Usage
- Create a sketch either from the
Sketcher Workbench or through the 
PartDesign Workbench. - Press
Sketcher NewSketch, or
PartDesign Body followed by 
PartDesign NewSketch.
- Double click the created sketch to enter its edit mode.
- Draw a series of lines using
Create polyline. - Pick one of the lines, and use
Constrain vertical. - Pick one of the lines, and use
Constrain horizontal. - Pick the vertical line, and use
Constrain distance Y; assign a distance. - Pick the horizontal line, and use
Constrain distance X; assign a distance.
Notes
- Constraints are useful to create very precise profiles which can the be turned into solid extrusions by using the
PartDesign Pad or 
Part Extrude operations. - Constraints are only used within Sketches; other 2D objects such as those created with the
Draft Workbench do not understand about constraints; the latter are simply placed in 3D space, and their properties define their shape and position.
Sketcher
- General: Create sketch, Edit sketch, Map sketch to face, Reorient sketch, Validate sketch, Merge sketches, Mirror sketch, Leave sketch, View sketch, View section, Toggle grid, Toggle snap, Configure rendering order, Stop operation
- Sketcher geometries: Point, Line, Arc, Arc by 3 points, Circle, Circle by 3 points, Ellipse, Ellipse by 3 points, Arc of ellipse, Arc of hyperbola, Arc of parabola, B-spline by control points, Periodic B-spline by control points, B-spline by knots, Periodic B-spline by knots, Polyline, Rectangle, Centered rectangle, Rounded rectangle, Triangle, Square, Pentagon, Hexagon, Heptagon, Octagon, Regular polygon, Slot, Fillet, Corner-preserving fillet, Trim, Extend, Split, External geometry, Carbon copy, Toggle construction geometry
- Sketcher constraints:
- Geometric constraints: Coincident, Point on object, Vertical, Horizontal, Parallel, Perpendicular, Tangent, Equal, Symmetric, Block
- Dimensional constraints: Lock, Horizontal distance, Vertical distance, Distance, Radius or weight, Diameter, Auto radius/diameter, Angle, Refraction (Snell's law)
- Constraint tools: Toggle driving/reference constraint, Activate/deactivate constraint
- Sketcher tools: Select unconstrained DoF, Select associated constraints, Select associated geometry, Select redundant constraints, Select conflicting constraints, Show/hide internal geometry, Select origin, Select horizontal axis, Select vertical axis, Symmetry, Clone, Copy, Move, Rectangular array, Remove axes alignment, Delete all geometry, Delete all constraints
- Sketcher B-spline tools: Show/hide B-spline degree, Show/hide B-spline control polygon, Show/hide B-spline curvature comb, Show/hide B-spline knot multiplicity, Show/hide B-spline control point weight, Convert geometry to B-spline, Increase B-spline degree, Decrease B-spline degree, Increase knot multiplicity, Decrease knot multiplicity, Insert knot, Join curves
- Sketcher virtual space: Switch virtual space
- Additional: Sketcher Dialog, Preferences, Sketcher scripting
Document objects
- Core: App DocumentObject
- Basic: App FeaturePython, App GeoFeature, Part Feature, Mesh Feature, Fem FemMeshObject
- Internal shapes: Part TopoShape, Mesh MeshObject, Fem FemMesh
- Structure: App DocumentObjectGroup (Std Group), App Part (Std Part), App Link (Std LinkMake)
- Derived: Part Part2DObject, Sketcher SketchObject, PartDesign Body, PartDesign Feature
User documentation
- Getting started
- Installation: Download, Windows, Linux, Mac, Additional components, Docker, AppImage, Ubuntu Snap
- Basics: About FreeCAD, Interface, Mouse navigation, Selection methods, Object name, Preferences, Workbenches, Document structure, Properties, Help FreeCAD, Donate
- Help: Tutorials, Video tutorials
- Workbenches: Std Base, Arch, Assembly, CAM, Draft, FEM, Inspection, Mesh, OpenSCAD, Part, PartDesign, Points, Reverse Engineering, Robot, Sketcher, Spreadsheet, Start, Surface, TechDraw, Test Framework, Web
- Hubs: User hub, Power users hub, Developer hub