Snapping and Inference
Snapping puts your click on the geometry you were aiming at and leaves a constraint behind, so the alignment survives the next edit. Draw normally and watch the cursor: the editor works out the target continuously and shows you what it will take before you commit the click.
That second half is what separates this from drawing on a grid. A point that merely landed on another point drifts apart the moment the sketch moves. A point that is coincident with it never does.
Straightening as you draw
Draw a segment within about three degrees of horizontal or vertical and it lands exactly on the axis, with a horizontal or vertical constraint to keep it there.
A dashed guide appears while the inference is live, so you can see it happening before you commit the click. Aim further off the axis and it does not trigger. Three degrees is deliberate: wide enough to catch what you meant, narrow enough not to hijack a line you drew at five.
Snapping to what exists
Bring the cursor near something snappable and the editor shows what it will take. It resolves in a fixed order, and the first match wins.
| Priority | Target | What the click leaves behind |
|---|---|---|
| 1 | An existing point | The two share the point outright |
| 2 | The midpoint of a line or arc | A midpoint constraint |
| 3 | A quadrant of a circle | A point-on-curve constraint |
| 4 | Anywhere on a circle or arc | A point-on-curve constraint |
| 5 | Anywhere along a line | A point-on-line constraint |
| 6 | A vertex or edge of the host face | A coincident constraint |
| 7 | A principal axis | A point-on-axis constraint |
| 8 | A horizontal / vertical alignment guide | Nothing — it only aims the cursor |
Some of those entries snap to something that was never drawn. There is no point at the middle of a segment, but the editor offers it, and the constraint it leaves keeps your line at the middle when the segment changes length.
Why the constraint matters more than the snap
Every snapped click shows up in the constraint list as a chip you can delete. Open the list after you draw, read what the editor decided, and remove anything you did not want.
Snap well and a sketch is often nearly defined before you have typed a single dimension, because each snap removed a degree of freedom.
See also
- Geometric Constraints: the constraints a snap leaves behind
- How the Solver Works: what those constraints do to a sketch