Sketching and 2D Geometry
The sketch as a constrained system
A Fusion sketch is a 2D profile on a plane or planar face, and its power comes from constraints rather than from precise clicking. Geometric constraints (coincident, horizontal, vertical, parallel, perpendicular, tangent, equal, concentric, collinear, midpoint, symmetry, fix) relate geometry to geometry; dimensional constraints (linear, angular, radial, diameter) set sizes. Fully constrained geometry turns black, under-constrained geometry stays blue, and conflicting or over-constrained geometry is flagged. Sketch dimensions can be driven by user parameters, and the Sketch Palette controls grid, snap, constraint inference, and constraint visibility.
Concepts to know
Understand construction lines and how they serve as reference geometry. Know the projection tools (Project and Include) that bring model edges into a sketch, and that projected geometry is associated with the model. Learn profile detection: closed regions become selectable profiles, and open or self-intersecting loops fail. Know Trim, Extend, Offset, Fillet, and Mirror within the sketch environment, and how the origin point anchors a sketch for predictable modeling.
Traps
Questions often imply that adding a dimension always fully constrains a sketch, or that construction geometry can be used to create a solid. Another recurring trap is treating projected geometry as independent of the source edge. Watch for items claiming that a sketch must be closed to be extruded as a surface.
Rule of thumb
Constrain intent first (horizontal, vertical, symmetry, equal), then dimension last. A sketch anchored to the origin with symmetric constraints survives design changes far better than one positioned with many absolute dimensions.