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Constraints
A constraint is a statement that must stay true. The solver moves geometry until every statement holds at once, and moves it again whenever you change one.
Constraints appear in three places at the same time, all doing the same thing: the Selection section of the sidebar, the floating bar above the canvas (the first six only), and the context menu. Which ones appear depends entirely on what you have selected.
Once applied, a constraint says so on the drawing: a small glyph beside the geometry it holds. See constraint badges.
The selection recipes
The exact selection matters. Two points and nothing else offers Coincident and Distance. Two points plus a line offers Symmetric instead. Selecting a third thing by accident is the usual reason an expected button is missing.
Between two points
| Constraint | Select | Value |
|---|---|---|
| Coincident | 2 points | |
| Distance | 2 points | mm, seeded from the current spacing |
On one point
| Constraint | Select | Value |
|---|---|---|
| Fix | 1 point |
Fix pins a point in place. Most sketches want exactly one, to stop the whole drawing sliding around the plane.
On one line
| Constraint | Select | Value |
|---|---|---|
| Horizontal | 1 line | |
| Vertical | 1 line |
Between two lines
| Constraint | Select | Value |
|---|---|---|
| Parallel | 2 lines | |
| Perpendicular | 2 lines | |
| Equal | 2 lines | Matches their lengths |
| Angle | 2 lines | degrees, seeded from the current unsigned angle (0 to 180) |
| Offset | 2 lines | mm |
Offset is a compound. It applies Parallel plus a point-to-line distance from an endpoint of the first line to the second line, as a single undo step. Once the lines are parallel that perpendicular gap is the line-to-line offset, and the distance row is the one you edit afterwards.
Between a point and a line
| Constraint | Select | Value |
|---|---|---|
| Point on line | 1 point + 1 line | |
| Midpoint | 1 point + 1 line | |
| Distance to line | 1 point + 1 line | mm |
Involving circles and arcs
Circles and arcs are interchangeable everywhere below. This documentation calls either a round.
| Constraint | Select | Value |
|---|---|---|
| Point on | 1 point + 1 round | |
| Radius | 1 round | mm |
| Diameter | 1 round | mm, seeded at twice the current radius |
| Equal | 2 rounds | Matches their radii |
| Tangent | 2 rounds, or 1 line + 1 round |
Across a mirror line
| Constraint | Select | Value |
|---|---|---|
| Symmetric | 2 points + 1 line, exactly three things |
Dimensions
Five constraints carry an editable number: Distance, Distance to line, Angle, Radius, Diameter. These are the dimensions, and they are what makes the part parametric.
Every dimension is seeded from the geometry as it currently stands, so applying one locks in what you already see rather than jumping the drawing somewhere unexpected. Then you type the number you wanted.
The value is edited in the constraint docket, or by clicking its annotation on the canvas. Both fields accept arithmetic. Angles are shown and typed in degrees.
Applying a dimension clears the selection and focuses its new value field, so you can apply and type without touching the mouse.
Constraints you never ask for
Some are added on your behalf:
| Where | What |
|---|---|
| Rectangle tool | Two Horizontal and two Vertical |
| Shift while drawing | A Horizontal or a Vertical |
| Pencil tangent arc | A Tangent to the previous segment |
| Add midpoint | A Midpoint per selected line |
| Import from an image | Horizontal, vertical, coincident and equal-radius, inferred from the drawing |
| Import from a file | Horizontal and vertical only. A file is not a guess, so nothing else is inferred |
If the solver rejects one of these, the geometry is still created. You get the shape without the constraint rather than losing both.
Add midpoint
Not a constraint you select for. It drops a new point at the middle of every selected line and constrains it there, as one undo step. It is on the top bar, on the floating bar, and in the context menu, and is disabled unless a line is selected.
When nothing applies
If you have a selection and the Selection section reads No constraints for this combination, the combination has none. Check the count: three things selected when you meant two is the usual cause.
What happens after
Watch the status foot. Each constraint you add should reduce the degrees-of-freedom count. If the count does not move, the constraint was redundant, and its dot in the docket will say so. If something turns red, it conflicts. Its badge turns the same colour, on the geometry itself. See diagnosing a sketch.