Translations:Manual:Modeling for product design/7/en

Part Design objects are fully based on Sketches. A Sketch is a 2D object, made of linear segments (lines, arcs of circle or ellipses) and constraints. These constraints can be applied either on linear segments or on their endpoints or center points, and will force the geometry to adopt certain rules. For example, you can place a vertical constraint on a line segment to force it to stay vertical, or a position (lock) constraint on an endpoint to prohibit it to move. When a sketch has an exact amount of constraints that prohibits any point of the sketch to be moved anymore, we talk about a fully constrained sketch. When there are redundant constraints, that could be removed without allowing the geometry to be moved, it is called over-constrained. This should be avoided, and FreeCAD will notify you if such case occurs.