What is a sketch in CAD and why are constraints important?

A sketch is a 2D profile used as the foundation for creating 3D features like extrusions, revolves, and sweeps. Sketch constraints (horizontal, vertical, coincident, equal, etc.) define geometric relationships between sketch entities, making the sketch fully defined. A fully constrained sketch is parametrically stable, meaning it responds predictably to dimension changes and doesn't shift unexpectedly during model updates.