Sketching
Constraints before dimensions
Inventor sketching rests on two constraint families. Geometric constraints, such as coincident, collinear, parallel, perpendicular, tangent, concentric, equal, and symmetric, define relationships. Dimensional constraints define size. A fully constrained sketch changes color and shows no degrees of freedom; an underconstrained sketch can still be extruded, which is a frequent source of confusion in questions.
Concepts a candidate must know
Understand construction geometry and its role in locating features. Know the difference between a closed profile and an open profile and which feature types accept each. Know how to project geometry from model edges, and how to use the sketch doctor and the constraint status readout. Understand inference and how automatic constraints are applied while drawing, plus how to override or delete them. Know pattern and mirror tools for sketch geometry, and the trim, extend, and offset tools.
Traps in questions
Questions often assume a sketch must be fully constrained to be used. It does not. Another trap confuses projected geometry with ordinary sketch geometry; projected edges update with the model, which is desirable but can create unexpected dependencies. Watch for items about shared sketches used by multiple features, and about the difference between deleting a constraint and deleting geometry. Also expect questions on the coordinate system of a sketch and how the sketch plane orientation affects extrusion direction.
Worked example
Suppose a rectangular profile shows two remaining degrees of freedom. Adding a horizontal constraint to one edge and a vertical constraint to an adjacent edge removes rotation, and a single dimension fixes size, leaving the rectangle fully constrained. Rule of thumb: remove rotational freedom first, then size.